Printing system and method

ABSTRACT

There is disclosed a method of printing onto the surface of a substrate, which method comprises i) coating a donor surface with a monolayer of particles, ii) treating the substrate surface to render at least selected regions tacky, and iii) contacting the substrate surface with the donor surface to cause particles to transfer from the donor surface only to the tacky regions of the substrate surface. After printing on a substrate, the donor surface returns to the coating station where the continuity of the monolayer is restored by recovering with fresh particles the regions of the donor surface exposed by the transfer of particles to the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national phase application filed under 35U.S.C. § 371 of International Application No. PCT/IB2016/053145, filedMay 27, 2016, designating the United States and claiming priority toBritish Patent Application No. 1509080.6, filed May 27, 2015; BritishPatent Application No. 1514618.6, filed Aug. 17, 2015; British PatentApplication No. 1514619.4 filed Aug. 17, 2015; and British PatentApplication No. 1603997.6, filed Mar. 8, 2016.

FIELD

The present disclosure relates to a printing system and method, and inparticular to a system and method capable of applying to a substrate alayer having a metallic appearance.

BACKGROUND

Of the numerous systems that have been proposed in the past for printingon a substrate, such as paper, card of plastics film, the system thatbears the closest resemblance to the system proposed herein is foilimaging, which falls into two broad categories. In hot foil blocking,also known as foil stamping, a heated die is stamped onto a foil that isplaced against the substrate. The foil has a coating, often of metal,and the application of heat and pressure causes the coating to adhere tothe substrate so as to leave the design of the die on the substrate. Atthe same time, the metal coating is removed to leave behind on the foila depleted region of the corresponding shape. Foil fusing or cold foilstamping is a related process avoiding the need for a die, wherein thefoil is bonded to an image area that is covered by an adhesive. Theadhesive image can be created by indirect printing, using printingplates or cylinders, as in offset, flexographic and gravure printers,using printing screens, as in serigraphic printers, or by directprinting, using image specific patterns, as in digital printers. Forexample of the latter, an adhesive can be applied to the substrate(e.g., by ink jetting) and, if needed, subsequently be activated (e.g.,by heat) to adhere to the foil, hence binding it to the substrate in thedesired pattern. Such foils typically comprise, layered in the followingorder, a carrier film, a release layer, enabling the separation of afollowing pigment or metal layer upon impression, and an adhesive layerfacilitating the attachment of the preceding color-imparting layer tothe printing substrate. Additional layers can be intercalated in thisbasic structure, such as a lacquer between a release layer and a metallayer, for example. Though such metal foils can be tens of micrometersthick, the thickness of the fully continuous metal layer or film in suchlaminated foils is generally of a few micrometers, typically less thanone, some metal foils even providing a thin integral metal coat of lessthan one hundred nanometers.

One of the main disadvantages of foil stamping and fusing is the largeamount of foil that is wasted during each stamp/fuse process, as anyfoil area that is not transferred to form the desired image on thesubstrate cannot be recovered for successive prints. Since foils,especially metal foils are expensive, foil imaging processes arerelatively high cost methods, as typically a roll of foil can only beused once and, when discarded, only a small fraction of the coating willhave been used.

OBJECT

The present disclosure seeks inter alia to provide a printing method andsystem that represent an effective alternative to foil imaging but thatis more cost effective and environmentally friendly, possibly providingdifferent physical properties to printed matter. Additionally, themethod herein disclosed may be used for the preparation of coatedsubstrates.

SUMMARY

According to a first aspect of the disclosure, there is provided amethod of printing onto a surface of a substrate, which comprisesproviding a donor surface, passing the donor surface through a coatingstation from which the donor surface exits coated with a monolayer ofindividual particles, and repeatedly performing the steps of:

-   -   (i) treating the substrate surface to render the affinity of the        particles to at least selected regions of the substrate surface        greater than the affinity of the particles to the donor surface,    -   (ii) contacting the substrate surface with the donor surface to        cause particles to transfer from the donor surface only to the        treated selected regions of the substrate surface, thereby        exposing regions of the donor surface from which particles are        transferred to corresponding regions on the substrate; and    -   (iii) returning the donor surface to the coating station to        render the particle monolayer continuous in order to permit        printing of a subsequent image on a substrate surface.

It will be appreciated that as in the present printing method theparticles form a monolayer on the donor surface, the particlestransferred therefrom also form a monolayer on the selected regions ofthe substrate surface. The regions of the substrate suitably treated maybe said to have a receptive layer.

The above method can be used repeatedly to create multiple copies of asame image or of different images on the relevant surfaces of one ormore substrates. A series of identical images printed on a samesubstrate is typically referred to as a “print job”.

The method herein disclosed may further include a cleaning step, duringwhich particles remaining on the donor surface after contacting thesubstrate are removed from the donor surface, so that prior to the nextpassage through the cleaning station the donor surface is substantiallydevoid of particles. Such cleaning step may be performed during eachprinting cycle or periodically, for instance in between print jobs,changes of particles and the like. A printing cycle corresponds to thetime period in-between subsequent passing of a reference point on thedonor surface through the coating station, such passage resulting fromthe donor surface being movable with respect to the coating station.

The donor surface coated with particles is used in a manner analogous tothe foil used in foil imaging. However, unlike foil imaging, the damagecaused to the continuity of the particle layer on the donor surface byeach impression can be repaired by re-coating only the exposed regionsof the donor surface from which the previously applied layer has beenstripped by transfer to the selected regions of the substrate.

The reason that the particle layer on the donor surface can be repairedafter each impression is that the particles are selected to adhere tothe donor surface more strongly than they do to one another. Thisresults in the applied layer being substantially a monolayer ofindividual particles. The term “monolayer”, defined more rigorouslyherein-below, is used herein to describe a layer in which—ideally—eachparticle has at least a portion that is in direct contact with the donorsurface prior to impression and at least a portion in contact with thesubstrate after impression. While some overlap may occur betweenparticles contacting any such surface, the layer may be only oneparticle deep over a major proportion of the area of the surface. Thisoccurs for the same reason that an adhesive tape, when used to pick up apowder from a surface, will only pick up one layer of powder particles.When the adhesive tape is still fresh, the powder will stick to theadhesive until it covers the entire tape surface. However, once theadhesive has been covered with powder, the tape cannot be used to pickup any more powder because the powder particles will not stick stronglyto one another and can simply be brushed off or blown away from thetape. Similarly, the monolayer herein is formed from the particles insufficient contact with the donor surface and is therefore typically asingle particle thick. Contact is said to be sufficient when it allowsthe particle to remain attached to the donor surface at the exit of thecoating station, e.g., following surplus extraction, burnishing, or anyother like step, some of which will be described in more detail below,by way of example.

Taking, for example, a platelet shaped particle contacting the donorsurface over most of its planar face (e.g., being substantiallyparallel), the resulting thickness of the monolayer (in the directionperpendicular to the surface) would approximately correspond to thethickness of the particle, hence the average thickness of the monolayercan be approximated by the average thickness of the individual particlesforming it. However, as there could be partial overlaps between adjacentparticles, the thickness of the monolayer can also amount, in someplaces, to a low multiple of the dimension of the constitutingparticles, depending on the type of overlap, for instance on therelative angles the particles may form with one another and/or with thedonor surface and/or the extent of the overlap. A monolayer maytherefore have a maximum thickness (7) corresponding to about one-fold,or about two-fold, or about three-fold, or any intermediate value, of athinnest dimension characteristic to the particles involved (e.g., thethickness of the particles for flake shaped ones or essentially theparticle diameter for spherical ones). The thinnest characteristicdimension of a particle, or population thereof, may generally beestimated by microscope techniques, for instance from SEM or SEM-FIBimages, and can be quantitatively determined for each particle, or forthe entire field of view of the image.

Because the layer is a monolayer mosaic of particles, if the surface onentering the coating station already carries a particle layer which isdiscontinuous (because particles have been stripped from selectedregions of a previously applied continuous layer), then the depletedregions alone can be replenished with particles without depositing freshparticles on those regions of the previously applied layer that arestill intact. However, parts of the monolayer coating that are not usedin one printing cycle may be removed from the donor surface (andpossibly recycled) and the donor surface may be cleaned before a newmonolayer is applied for the next printing cycle. This could bedesirable if the physical interactions that occur during imageimpression somehow modify the properties of the donor surface, resultingin a ghost image being printed during the following operating cycle. Acleaning, and a possible treatment step, would in such a case ensurethat the donor surface is restored to its original state at thecommencement of each operating cycle.

For a relatively light effect or matte appearance, the area coverage bythe mosaic of particles can be smaller (e.g., below 50%) than for glossyor mirror-like appearance. For such high gloss visual appearance, themosaic of particles can sufficiently cover the target surface so thatthe reflection resulting from the particles transferred to the substrateis suitable for the desired visual effect. For the same effect, andassuming all other parameters are equivalent, particles having arelatively higher reflectivity and/or more parallel orientation with theprinting substrate may only need to cover a smaller percent area of thetarget surface than particles having a relatively lower reflectivityand/or a more random/less parallel orientation relative to thesubstrate. The relative reflectivity relates to the properties of therespective particles and can also be affected by the characteristics ofthe substrate, features of the background image, and any suchconsiderations readily understood by persons skilled in the art of metalprinting. By “sufficient” covering, it is meant that the coat ofparticles on the relevant substrate regions will be devoid of defectsperceptible to the naked eye, such as discontinuities or holes in themosaic of particles that would expose the substrate surface to an extentvisually detectable and detrimental to the intended effect. Having atleast 50% of the area of the surface of the selected substrate region(s)to be coated, or at least 60%, or at least 70% of this area covered byparticles may be sufficient coverage (i.e., providing for a sufficientlycontinuous layer of particles).

For high end mirror-like appearance substantially the whole of theselected surfaces of the substrate to be coated may need to be covered.By “substantially” covering, it is meant that, as for sufficientcovering, the coat of particles on the relevant substrate regions willbe devoid of visible defects, such as discontinuities or holes in themosaic of particles that would expose the substrate surface to an extentdetectable by the naked eye. Having at least 80% of the area of thesurface of the selected substrate region(s) to be coated by particles,or at least 85%, or at least 90% or at least 95% of the area covered byparticles is considered a substantial coverage (i.e., providing for asubstantially continuous layer of particles).

As such sufficiently or substantially continuous layers of particles onthe substrate surface, or part thereof, results from the transfer ofsame particles from the donor surface, it is to be understood that asufficiently coated donor surface will correspondingly have at least50%, or at least 60%, or at least 70% of its area covered by particles,while a substantially fully coated donor surface will correspondinglyhave at least 80%, or at least 85%, or at least 90% or at least 95% ofits area covered by particles. As mentioned, for lower end effect, anarea coverage of less than 50% can be satisfactory. Thus depending onthe desired effect and on the particles involved, a monolayer of up to50% area coverage can be used according to the present teachings.Depending on the surface being considered, the percent area coverage canbe of at least 10%, or at least 20% or at least 30%.

For matte effects, the particle can be selected to provide such a lookor can be oriented on the printing substrate in a manner providing sucheffect. As readily understood, particles being non-parallel with thesurface of a substrate, even if being reflective, may diffract light ina way resulting in an overall matte effect. A matte effect can thereforebe achieved by using a substrate having a relatively rough surface, arelatively thin receptive layer maintaining the roughness of theparticle reception surface or any other substrate with a relativelythick receptive layer, the particle reception surface being patterned toprovide for a surface roughness providing such “non-parallel” or randomorientation of the particles and matte effect.

The percentage of an area covered by particles out of a specific targetsurface can be assessed by numerous methods known to skilled persons,including by determination of optical density possibly in combinationwith the establishment of a calibration curve of known coverage points,by measurement of transmitted light if either the particles or thesubstrate are sufficiently transparent, or conversely, by measurement ofreflected light, for instance if the particles are reflective.

As used in the specification, a preferred method of determining thepercentage area of a surface of interest covered by particles is asfollows. Squared samples having 1 cm edges are cut from the surfacebeing studied (e.g., from the donor surface or from the printedsubstrate). The samples are analyzed by microscopy (either laserconfocal microscopy (Olympus, LEXT OLS30ISU) or optical microscopy(Olympus BX61 U-LH100-3)) at a magnification of up to ×100 (yielding afield of view of at least about 128.9 μm×128.6 μm). At least threerepresentative images are captured in reflectance mode for each sampleprinted on an opaque substrate (e.g., paper). The captured images wereanalyzed using ImageJ, a public domain Java image processing programdeveloped by the National Institute of Health (NIH), USA. The images aredisplayed in 8-bit, gray scale, the program being instructed to proposea threshold value of reflectance differentiating between the reflectiveparticles (lighter pixels) and the interstices that may exist betweenneighboring or adjacent particles (such voids appearing as darkerpixels). A trained operator may adjust the proposed threshold value, ifneeded, but typically confirms it. The image analysis program thenproceed to measure the amount of pixels representing the particles andthe amount of pixels representing the uncovered areas of theintra-particle voids, from which the percent area of coverage can bereadily calculated. Measurements done on the different image sections ofthe same sample are averaged. When the samples are printed on atransparent substrate (e.g., a translucent plastic foil), a similaranalysis can be done in transmittance mode, the particles appearing asdarker pixels and the voids as lighter ones. Results obtained by suchmethods, or by any substantially similar analytical techniques known tothose of skill in the art, are referred to as optical surface coverage,which can be expressed in percent or as a ratio.

If printing is to take place on the entire surface of the substrate, thereceptive layer, which may for example be an adhesive, may be applied tothe substrate by a roller before it is pressed against the donorsurface. As the regions of a substrate suitably treated to receiveparticles being transferred from the donor surface, also said to harbora corresponding receptive layer, can be an adhesive or act as theadhesive side of the tape illustration, the receptive layer may oftenalso be referred to as an adhesive, this should not however be construedas limiting.

If printing is only to take place on selected regions of the substrate,on the other hand, then it is possible to apply the adhesive by anyconventional printing method, for example by means of a die or printingplates, or by jetting the receptive layer onto the surface of thesubstrate. As a further possibility, it is possible to coat the entiresurface of the substrate with an activatable receptive layer that isselectively rendered “tacky” by suitable activation means. Whetherselectively applied or selectively activated, the receptive layer insuch case forms a pattern constituting at least part of the image beingprinted on the substrate.

The term “tacky” is used herein only to indicate that the substratesurface, or any selected region thereof, has sufficient affinity to theparticles to separate them from the donor surface and/or to retain themon the substrate, when the two are pressed one against the other at animpression station, and it need not necessarily be tacky to the touch.To permit the printing of patterns in selected regions of the substrate,the affinity of the receptive layer, activated if needed, towards theparticles needs to be greater than the affinity of the bare substrate tothe particles. In the present context, a substrate is termed “bare” iflacking a receptive layer or lacking a suitably activated receptivelayer, as the case may be. Though the bare substrate should for mostpurposes have substantially no affinity to the particles, to enable theselective affinity of the receptive layer, some residual affinity can betolerated (e.g., if not visually detectable) or even desired forparticular printing effects.

The receptive layer may, for instance, be activated by exposure toradiation (e.g., UV, IR and near IR) prior to being pressed against thedonor surface. Other means of receptive layer activation includetemperature, pressure, moisture (e.g., for rewettable adhesives) andeven ultra sound, and such means of treating the receptive layer surfaceof a substrate can be combined to render tacky the compatible receptivelayer.

Though the nature of the receptive layer being applied to the surface ofthe substrate may differ, among other things, from substrate tosubstrate, with the mode of application and/or the selected means ofactivation, such formulations are known in the art and need not befurther detailed for an understanding of the present printing method andsystem. Briefly, thermoplastic, thermosetting or hot-melt polymerscompatible with the intended substrate and displaying sufficienttackiness, relative affinity, to the envisioned particle, optionallyupon activation, can be used for the implementation of the presentdisclosure. Preferably the receptive layer is selected so that it doesnot interfere with the desired printing effect (e.g., clear,transparent, and/or colorless).

A desired feature of the suitable adhesives relates to the relativelyshort time period required for activating the receptive layer, i.e.selectively changing the receptive layer from a non-tacky state to atacky state, increasing the affinity of the selected region of thesubstrate so that it becomes sufficiently attached to the particles toseparate them from the donor surface. Fast activation times enable thereceptive layer to be used in high-speed printing. Adhesives suitablefor implementation of the present disclosure are preferably capable ofactivation within a period of time no longer than the time it takes thesubstrate to travel from an activating station to the impressionstation.

In some embodiments, activation of the receptive layer can take placesubstantially instantaneously at the time of the impression. In otherembodiments, the activation station or step may precede the impression,in which case the receptive layer can be activated within a time periodof less than 10 seconds or 1 second, in particular in a time period ofless than about 0.1 second and even less than 0.01 second. This timeperiod is referred to herein as the receptive layer's “activation time.”

A receptive layer requiring activation to gain sufficient affinity,needs to remain in such state long enough to at least allow transfer ofthe particles from the donor surface to the printing substrate beforethe receptive layer loses its tackiness. In some printing systems thereceptive layer may be applied on each substrate “in-line” upstream ofthe impression station, so as to be deposited in tacky form. The periodof time during which the receptive layer is sufficiently tacky for theintended system is described herein as the “open time” of the receptivelayer. Suitable adhesives exhibit an open time commensurate with thetransfer conditions and/or the subsequent stations or steps of theparticular printing system or process. If, for instance, the printingsystem is to comprise a plurality of coating stations, it is desiredthat the receptive layer selectively activated at or prior to reaching afirst station revert to a non-tacky state before it reaches a secondcoating station at which the treatment of the substrate could be appliedto a different portion, most likely to adhere to particles havingdifferent properties (e.g., different colors). In some printing systems,the receptive layer may be constantly tacky, its “infinite” open timebeing de facto limited by the subsequent application of the particles,which block its later ability to further adhere to additional particles.

Generally open times of activated adhesives are suitably of at leastfrom about 0.01 second to a few seconds (e.g., up to 10 seconds), thoughlonger open times (e.g., of a few minutes) may be suitable for certainapplications and “infinite” open times can be suitable when thereceptive layer is being applied in tacky stage (in other words,“already activated”) in a desired pattern upstream of the impressionstation (e.g., the substrate is treated by deposition of a tackymaterial on its surface).

Independently of the printing method having been used to apply oractivate the receptive layer to the image receiving side of a substrate,such application or activation being optionally selective so as to forma desired pattern, a suitable receptive layer may be selected asfollows.

As already mentioned, a suitable receptive layer needs have sufficientaffinity with the particles due to form the monolayer according to thepresent teachings. This affinity, which can be alternatively consideredas an intimate contact between the two, needs to be sufficient to retainthe particles on the surface of the receptive layer and can result fromthe respective physical and/or chemical properties of the layer and theparticles. For instance, the receptive layer may have a hardnesssufficiently high to provide for satisfactory print quality, butsufficiently low to permit the adhesion of the particles to the layer.Such optimum range can be seen as enabling the receptive layer to be“locally deformable” at the scale of the particles, so as to formsufficient contact. Such affinity or contact can be additionallyincreased by chemical bonding. For instance, the materials forming thereceptive layer can be selected to have functional groups suitable toretain the particles by reversible bonding (supporting non-covalentelectrostatic interactions, hydrogen bonds and Van der Waalsinteractions) or by covalent bonding. Likewise, the receptive layerneeds be suitable to the intended printing substrate, all aboveconsiderations being known to the skilled person.

The receptive layer can have a wide range of thicknesses, depending forexample on the printing substrate and/or on the desired printing effect.A relatively thick receptive layer can provide for an “embossing”aspect, the design being raised above the surface of the surroundingsubstrate. A relatively thin receptive layer can follow the contour ofthe surface of the printing substrate, and for instance for roughsubstrates enable a matte aspect. For glossy aspect, the thickness ofthe receptive layer is typically selected to mask the substrateroughness, so as to provide an even surface. For instance, for verysmooth substrates, such as plastic films, the receptive layer may have athickness of only a few tens of nanometers, for example of about 100 nmfor a polyester film (for instance a polyethylene terephthalate (PET)foil) having a surface roughness of 50 nm, smoother PET films allowingto use even thinner receptive layers. Substrates having rougher surfacesin the micron, or tens of microns, range will benefit of a receptivelayer having a thickness in the same size range or order of size range,if glossy effect, hence some leveling/masking of substrate roughness isdesired. Therefore depending on the substrate and/or desired effect, thereceptive layer can have a thickness of at least 10 nm, or at least 50nm, or at least 100 nm, or at least 500 nm, or at least 1,000 nm. Foreffects that can be perceptible by tactile and/or visual detection, thereceptive layer may even have a thickness of at least 1.2 micrometers(μm), at least 1.5 μm, at least 2 μm, at least 3 μm, at least 5 μm, atleast 10 μm, at least 20 μm, at least 30 μm, at least 50 μm, or at least100 μm. Though some effects and/or substrates (e.g., cardboard, carton,fabric, leather and the like) may require receptive layers having athickness in the millimeter range, the thickness of the receptive layertypically does not exceed 800 micrometers (μm), being at most 600 μm, atmost 500 μm, at most 300 μm, at most 250 μm, at most 200 μm, or at most150 μm.

After printing has taken place, namely after the particles aretransferred from the donor surface to the tacky regions of the treatedsubstrate surface (i.e., the receptive layer) upon pressing, thesubstrate may be further processed, such as by application of heatand/or pressure, to fix or burnish the printed image and/or it may becoated with a varnish (e.g., colorless or colored transparent,translucent, or opaque overcoat) to protect the printed surface and/orit may be overprinted with an ink of a different color (e.g., forming aforeground image). While some post-transfer steps may be performed onthe entire surface of the printed substrate (e.g., further pressure),other steps may be applied only to selected parts thereof. For instance,a varnish may be selectively applied to parts of the image, for instanceto the selected regions coated with the particles, optionally furtherimparting a coloring effect.

Such elective overcoats, which may cover and optionally seal at leastregions of the substrate coated with the monolayer of particles,advantageously can satisfactorily adhere to the particles and/or becompatible with the receptive layer underneath said monolayer.Attachment of the overcoat to the particles can be optionally enhancedby physical treatment of the surface with plasma or a corona discharge.In embodiments wherein the receptive layer requires post-impressiontreatment, the overcoat preferably enables such treatment. If, forexample, a particular receptive layer requires final UV-curing followingtransfer of particles thereupon, an overcoat applied upon the particlesneeds to permit the transmission of the UV radiation necessary toachieve such curing.

Any device suitable to perform any such post-transfer step can bereferred to as a post-transfer device (e.g., a coating device, aburnishing device, a pressing device, a heating device, a curing device,and the like). Post-transfer devices may additionally include anyfinishing device conventionally used in printing systems (e.g., alaminating device, a cutting device, a trimming device, a punchingdevice, an embossing device, a perforating device, a creasing device, abinding device, a folding device, and the like). Post-transfer devicescan be any suitable conventional equipment, and their integration in thepresent printing system will be clear to the person skilled in the artwithout the need for more detailed description.

The particles may include any material to be applied to the surface ofthe substrate. In particular, suitable material for the particles mayinclude compounds providing for a desired printing effect and encompasscoloring agents (e.g., pigments and dyes) generally bound to a polymericresin (e.g., a non-thermoplastic polymer) and any other material havinga desired printing effect (e.g., providing a metallic look or aglittering effect etc.).

As the effect to be achieved is similar to foil imaging, such as usedfor instance for metal printing, then the particles may be grains orflakes of metals, such as aluminum, copper, iron, zinc, nickel, tin,titanium, gold or silver, or alloys, such as steel, bronze or brass, andlike compounds predominantly including metals. In addition to being madeof real metals, suitable particles can be made of compounds providingfor a similar visual effect (e.g., made of a polymeric or ceramicmaterial having a metallic appearance). Such “metal-like” materials aretypically predominantly non-metallic, a metal coat optionally serving toprovide the light reflectivity that may be perceived as metallic. By wayof example, particles manufactured using the PVD (physical vapordeposition) method, wherein a polymer foil is vapor coated in vacuumwith the metal of interest (including chrome, magnesium and theabove-mentioned exemplary metals) and thereafter crushed to formindividual flakes, may form metal-like particles if the polymer backboneis retained and can be deemed “metallic” if the polymer is eliminatedfollowing the deposition process.

If the effect to be achieved includes a glittering and/or a pearlescentand/or a nacreous effect, synthetic high polymers (including for examplemulti-layered structures of polyacrylates), magnesium fluoride,muscovite, aragonite, rutile or anatase titanium dioxide, mica compounds(typically coated with metal oxides) and the like can be used for theparticles. All of the foregoing exemplary particles, including thegenuinely metallic particles though collectively termed for simplicity“metal-looking” particles (i.e., providing a visual effect similar to ametallic compound), may be coated or uncoated.

The coating of the particles, which can be applied by physical but moretypically chemical means, can, among other things, reduce or prevent theparticles sticking to one another (e.g., as achievable with anti-cakingagents and the like), increase the repulsion between the particles(e.g., as achievable by increasing the charge of the particles), protectthe particles from undesired chemical modification (e.g., reduce,prevent or delay the oxidation of metals and alloys or any otherdeleterious aging of the metal-looking particles) or further increasethe affinity of the particles to the donor surface or to the selectedregions of the substrate, as desired (e.g., modify the hydrophobicity ofthe coats/surfaces).

Without wishing to be bound by theory, it is believed that the particlesmay have a tendency to adhere to the donor surface not only on accountof the interaction between two different hydrophobic surfaces but alsoas a result of a charge based interaction. It may therefore be possibleto enhance the affinity between the particles and the donor surface bysubjecting the donor surface for a conditioning treatment, such asexposure to a corona discharge or application of a chemical treatmentsolution. Any such treatment can be performed by a suitable conditioningdevice.

Particles suitable for a printing system and method according to thepresent teachings may for example be coated by one or more of i) anunmodified or modified carboxylic acid or fatty acid, the carboxylicacid selected from the group comprising, but not limited to, stearicacid, palmitic acid, behenic acid, benzoic acid, and oleic acid; ii) anoily substance selected from the group comprising, but not limited to,vegetal oils, such as linseed oil, sunflower oil, palm oil, soya oil,and coconut oil; mineral oils and synthetic oils; and iii) an oxidewhich may be of same or different material as the core particle beingcoated. For instance, aluminum particles may be coated with an aluminumoxide or a silicon dioxide, and mica particles may be coated withtitanium dioxide and iron oxide, for example. The particle coating mayoptionally modify the coloring effect of the core particle, this can beachieved for instance with some metal oxides or with pigmented polymers(e.g., a polyacrylate containing inorganic or organic absorptionpigments). Such coloring effect can also result from the choice of thecore particle, or from a partial oxidation of the same.

Whether colored polymers or metal-looking, the particles may provide,once transferred to the printing substrate, for a glossy or matte image,and for any other type of desired effect in accordance with the selectedparticles.

According to a further aspect of the disclosure, there is provided aprinting system comprising:

a continuously circulating endless donor surface,

a coating station for applying particles to the donor surface, the donorsurface bearing a monolayer coating of individual particles on exitingthe coating station,

a treatment station at which a substrate surface is treated to produceselected regions of the substrate surface having an affinity to theparticles on the donor surface that is greater than the affinity of theparticles to the donor surface, and

an impression station at which the substrate surface contacts the donorsurface to cause particles to transfer from the donor surface to onlythe selected regions of the substrate surface, thereby exposingcorresponding regions of the donor surface,

wherein after passing through the impression station, the donor surfacereturns, during operation, to the coating station for the layer ofparticles to be rendered continuous by the application of freshparticles to the exposed regions of the donor surface.

It is possible for the coating station to be static, while the donorsurface is cyclically movable, being either the outer surface of arotatable drum, or of an endlessly circulating belt or even of a platemoving back and forth so as to ensure his surface is exposed to thecoating station from edge to edge. All such forms of donor surfaces canbe said to be movable (e.g., cyclically, endlessly or repeatedlymovable) with respect to the coating station where particles can beapplied to the donor surface, the donor surface bearing a monolayercoating of individual particles on exiting the coating station (havingcompleted a cycle). The passing of the donor surface through a coatingstation or the donor surface being continuously circulating therein canbe achieved by any such movable donor surface.

In some embodiments, the coating station comprises a supply of particlessuspended in a fluid, the particles adhering more strongly to the donorsurface than to one another, an application device for applying thefluid to the donor surface in a manner to cause the particles suspendedin the fluid to adhere to the donor surface so as to form a particlecoating on the surface, and a surplus extraction system operative toextract fluid and to remove surplus particles that are not in directcontact with the surface, so as to leave only a monolayer of particlesadhering to the donor surface on exiting the coating station.

The application device may comprise a spray head for spraying the fluidand suspended particles directly onto the donor surface. Alternatively,the application device may comprise a rotatable applicator operative towipe the fluid and suspended particles onto the surface. When theparticles are applied by the application device in a liquid fluid, thedevice may further comprise, if needed, a drying element enabling theparticle coating to be substantially dry by the time it reaches asubsequent station. In some embodiments, the particles on the donorsurface are substantially dry upon contacting of the receptive layer onthe substrate at the impression station.

In the present disclosure, the term “suspended in” and its variations isto be understood as “carried by” and like terms, not referring to anyparticular type of mixture of materials of same or different phase.

The printing system may be an offline, stand-alone machine, or may bein-line with a printing press and/or other finishing units. Forinstance, the printing system according to the present disclosure canserve as one station or module in offset, flexographic, gravure,serigraphic and digital printing presses.

Additionally, a printing system according to the present teachings maycomprise, upstream of the coating station, more than a station forapplying a receptive layer or treating the substrate to form it. Forinstance, the system may include a station for applying a backgroundimage, the receptive layer being subsequently applied or activatedthereupon to form (following impression) a foreground image on thepreviously applied background. Conversely, the receptive layer can forma background image, whereas a foreground image is thereafter applied.The foreground and background images may form distinct parts of theimage to be printed, but may also overlap. Each of the foreground andbackground images, if both are desired for a particular image to beprinted, can be applied by any printing system.

For instance, a background image can be applied at a first station forflexographic printing of a colored surrounding, and a receptive layercan be applied at a second station, in a manner that may either at leastpartially overlap with the background image or in a separatenon-overlapping region of the substrate.

The above-described printing method and printing system can have a widerange of uses in commercial and decorative printing, including in thepublishing and packaging industry, where they can serve, for instance,to create decorative finishes (e.g., in luxury packaging) andanti-counterfeiting measures (e.g., in bank notes).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 depicts schematically an embodiment of a printing system of thepresent disclosure;

FIG. 2 is a view similar to that of FIG. 1 showing an embodiment havingan alternative particle application device that includes a rotatingapplicator;

FIG. 3 schematically illustrates an exemplary embodiment of a coatingstation for a printing system according to the present disclosure;

FIG. 4 schematically illustrates an exemplary embodiment of a printingsystem having a plurality of impression stations according to thepresent disclosure;

FIG. 5A is a picture on a black background paper substrate bearing analternative pattern of a receptive layer applied by flexographicprinting, the substrate being yet to be fed into an impression stationaccording to the present disclosure;

FIG. 5B is a picture of the same pattern as shown in FIG. 5A applied ona white paper substrate, on exiting the impression station;

FIG. 5C is a picture of the same pattern as shown in FIG. 5A applied ona transparent plastic substrate, on exiting the impression station;

FIG. 5D is a picture of the same substrate as shown in FIG. 5A, onexiting the impression station;

FIGS. 6A to 6D are details of FIGS. 5A to 5D, respectively, drawn to anenlarged scale;

FIG. 7A is an image captured by confocal microscopy of a metallisedsubstrate surface produced by hot foil stamping;

FIG. 7B is an image captured by confocal microscopy of a metallisedsubstrate surface produced by offset printing;

FIG. 7C is an image captured by confocal microscopy of a metallisedsubstrate surface produced by gravure printing;

FIG. 7D is an image captured by confocal microscopy of a metallisedsubstrate surface produced by flexographic printing;

FIG. 7E is an image captured by confocal microscopy of a metallisedsubstrate surface produced using a printing system and method of thepresent disclosure;

FIG. 7F is an image captured by confocal microscopy of a particle coateddonor surface used in the printing system and method of the presentdisclosure;

FIGS. 8A and 8B are schematic cross-section illustrations of printedconstructions that can be produced using a printing system and method ofthe present disclosure;

FIG. 9A is a schematic cross-section illustration of a conventionalprint construction as can be obtained by metal foil printing;

FIG. 9B is a schematic cross-section illustration of a printconstruction as can be obtained by conventional printing using a typicalbinder-containing metal ink wherein the particles display a non-leafingbehavior; and

FIG. 9C is a schematic cross-section illustration of a printconstruction as can be obtained by conventional printing using a typicalbinder-containing metal ink wherein the particles display a leafingbehavior.

DETAILED DESCRIPTION

The ensuing description, together with the figures, makes apparent to aperson having ordinary skill in the pertinent art how the teachings ofthe disclosure may be practiced, by way of non-limiting examples. Thefigures are for the purpose of illustrative discussion and no attempt ismade to show structural details of an embodiment in more detail than isnecessary for a fundamental understanding of the disclosure. For thesake of clarity and simplicity, some objects depicted in the figures maynot be drawn to scale.

Overall Description of the Printing System

FIG. 1 shows a drum 10 having an outer surface 12 that serves as a donorsurface. As the drum rotates clockwise, as represented by an arrow, itpasses beneath a particle coating apparatus 14 where it acquires amonolayer coating of fine particles. Next, the surface passes through animpression station 18 where a printing substrate 20 is compressedbetween the drum 10 and an impression cylinder 22. The side of theprinting substrate 20 to which the particles are transferred may bereferred to as an image-receiving surface and is designated 80 in FIGS.1 and 2. Selected regions of the surface of the printing substrate 20are rendered tacky, for example in one of the ways described below,prior to coming into contact with the donor surface 12. This causes themonolayer of fine particles to adhere to the tacky regions of thesubstrate and to separate from the donor surface 12. The regions on thedonor surface corresponding to the tacky areas or selected regions ofthe substrate bearing the receptive layer consequently become exposed,being depleted by the transfer of particles. The donor surface 12 canthen complete its cycle by returning to the coating apparatus 14 where afresh monolayer particle coating is applied only to the exposed regionsfrom which the previously applied particles were transferred to theselected regions of the substrate 20 in the impression station 18.

In the embodiment of FIG. 1, the substrate 20 carries a receptive layer(e.g., made of an adhesive) that is activated and rendered tacky inselected regions by exposure to radiation using as a treating station animaging system 16, described in greater detail below. In the embodimentof FIG. 2, on the other hand, prior to contacting the donor surface 12,the substrate 20 passes through a treating station 36 between a die 30and a roller 32. The die 30 has an embossed pattern that picks up anadhesive from an application roller 34 (e.g., an anilox roll) and placesreceptive layer 26 (e.g., an adhesive layer) on the substrate accordingto the die pattern. If printing is to cover the entire surface of thesubstrate 20, the die 30 may be replaced by a plain roller. The rollers30, 32 and 34 may form additional types of rotating treating stations36, and can for instance serve for the application and/or activation ofa receptive layer by offset, rotogravure, flexography or rotatingsilkscreen printing. A treating station, as illustrated by imagingsystem 16 can be referred to as a digital treating station, while atreating station as illustrated by rotating system 36 can be referred toas an analog treating station.

The above examples also illustrate two options for the preparation ofthe substrate bearing the active (already tacky, with sufficientaffinity to the particles) or “activatable” receptive layer or adhesive.In one case, such preparation can occur off-line, the printing systemonly requiring a substrate transport system able to feed such off-lineprepared substrates to the impression station, wherein the activationoccurs either downstream of the treating station wherein the receptivelayer was applied or at the impression station. In another case, theaddition of the receptive layer to the substrate or its activation canoccur in-line with the other steps of the printing process.

Additional methods for selectively applying or activating an adhesive orany other type of receptive layer on a printing substrate are known, maybe applicable as will be clear to the person skilled in the art and neednot be detailed herein, the two aforementioned methods beingnon-limiting examples. For instance, the receptive layer can be appliedat the treating station by silkscreen printing, and optionally furtheractivated at a downstream activation station, preceding the impressionstation. Activation can for example involve curing of the receptivelayer prior to the contacting of the particles. In some embodiments,curing (or further curing) may also serve as a post-transfer processingstep (e.g., improving the immobilization of the particles on a curablereceptive layer on the substrate).

The Particle Coating Apparatus

The particle coating apparatus 14 in the embodiment of FIG. 1 comprisesa plurality of spray heads 1401 that are aligned with each other alongthe axis of the drum 10 and only one is therefore seen in the section ofthe drawing. The sprays 1402 of the spray heads are confined within abell housing 1403, of which the lower rim 1404 is shaped to conformclosely to the donor surface leaving only a narrow gap between the bellhousing 1403 and the drum 10. The spray heads 1401 are connected to acommon supply rail 1405 which supplies to the spray heads 1401 apressurized fluid carrier (gaseous or liquid) having suspended within itthe fine particles to be used in coating the donor surface 12. If neededthe suspended particles may be regularly or constantly mixed, inparticular before their supply to the spray head(s). The particles mayfor instance be circulated in the coating apparatus within a flow raterange of 0.1 to 10 liter/minute, or in the range of 0.3 to 3 liter/min.The fluid and the surplus particles from the sprays heads 1401, whichare confined within a plenum 1406 formed by the inner space of thehousing 1403, are extracted through an outlet pipe 1407, which isconnected to a suitable suction source represented by an arrow, and canbe recycled back to the spray heads 1401. Though herein referred to asspray heads, any other type of nozzle or orifice along the common supplypipe or conduit allowing applying the fluid suspended particles areencompassed.

It is important to be able to achieve an effective seal between thehousing 1403 and the donor surface 12, in order to prevent the sprayfluid and the particles from escaping through the narrow gap that mustessentially remain between the housing 1403 and the donor surface 12 ofthe drum 10. Different ways of achieving such a seal are shownschematically in the drawing.

The simplest form of seal is a wiper blade 1408. Such a seal makesphysical contact with the donor surface and could score the appliedcoating if used on the exit side of the housing 1403, that is to say theside downstream of the spray heads 1401. For this reason, if such a sealis used, it is preferred for it to be located only upstream of the sprayheads 1401 and/or at the axial ends of the housing 1403. The terms“upstream” and “downstream” as used herein are referenced to points onthe donor surface 12 as it passes through the coating station.

FIG. 1 also shows how egress of the fluid within which the particles aresuspended from the sealing gap between the housing 1403 and the drum 10can be prevented without a member contacting the donor surface 12. Agallery 1409 extending in the present illustration around the entirecircumference of the housing 1403 is connected by a set of fine passages1410 extending around the entire rim of the housing 1403 to establishfluid communication between the gallery 1409 and the sealing gap.

In a first embodiment, the gallery 1409 is connected to a suction sourceof a surplus extraction system, which may be the same suction source asis connected to the outlet 1407 or a different one. In this case, thegallery 1409 serves to extract fluid passing through the gap before itexits the housing 1403. The low pressure also sucks off the drum 10 anyparticles that are not in direct contact with the donor surface 12 and,if the sprayed fluid is a liquid, it also sucks off surplus liquid to atleast partially dry the coating before it leaves the particle coatingapparatus 14.

Surplus liquid can alternatively and additionally be removed by mean ofa liquid extracting roller positioned on the exit side of the coatingapparatus. Such a roller, schematically illustrated as 1440 in FIG. 3,which has on its outer surface 1442 sponge-like liquid absorbingproperties (e.g., closed-cell foam), can be independently driven torotate at a speed and/or in a direction differing from the speed anddirection of drum 10 (only partially represented). The liquid extractingroller can contact the particles coated on the donor surface 12 andextract surplus liquid by drawing it within its fluid absorbing outersurface 1442, which is advantageously sufficiently smooth and even so asnot to affect the layer of particles retained on the donor surface priorto their selective transfer to the substrate 20. As the extractingroller 1440 continues to rotate following the absorption of the surplusliquid, it approaches a wiper 1444, or any other suitable mean,positioned so as to squeeze the roller and release the extracted liquidout of its absorbing surface. A suction inlet, schematically representedby arrow 1446, can be positioned adjacent to such wiper, so as to permitthe immediate removal of the liquid so extracted from the particlecoated donor surface and so forced out of the roller outer surface.Following such elimination of the removed liquid, the roller 1440 cancomplete its cycle, contacting again the donor surface and furtherextracting surplus liquid. Though illustrated in FIG. 3 as beinginternal to a coating station 14 (i.e. within partially representedplenum 1406 of housing 1403), a liquid extracting roller 1440, ifpresent, can alternatively be positioned downstream of the coatingstation, as long as it remains upstream of a station where liquidremoval is desired. The liquid extracting roller and its afore-describedassociated elements can be collectively referred to as a liquidabsorbing device.

As mentioned, the printing system may further comprise a dryer (e.g.,hot or cold air blower) on the exit side of the coating apparatus 14, orfurther downstream, so as to allow the particle coat to reach asubsequent station in substantially dry form.

In an alternative embodiment, the gallery 1409 is connected to a sourceof gas at a pressure higher than the pressure in the plenum 1406.Depending on the rate of fluid supply to the plenum through the sprayheads 1401 and the rate of extraction through the outlet 1407, theplenum 1406 may be at a pressure either above or below the ambientatmospheric pressure.

If the plenum is at sub-atmospheric pressure, then is suffices for thegallery 1409 to be at ambient atmospheric pressure, or indeed no galleryneed be present. In this case, because the pressure within the sealinggap will exceed the pressure in the plenum 1406, gas flow through thegap will be towards the interior of the housing with no risk of fluidegress.

If the plenum is at above atmospheric pressure, then the gallery 1409may be connected to a pressurized gas supply, preferably air. In thiscase, air will be forced into the sealing gap under pressure through thepassages 1410 and will split into two streams. One stream will flowtowards the plenum 1406 and will prevent egress of the fluid withinwhich the particles are suspended. That stream will also dislodge and/orentrain particles not in direct contact with the donor surface andassist in at least partially drying the coating if the carrier fluid isa liquid. The second stream will escape from the coating apparatuswithout presenting a problem as it is only clean air without anysuspended particles. The second gas stream may also assist in furtherdrying of the particle coating on the donor surface 12 before it leavesthe coating apparatus 14. If desired, the gas stream can be heated tofacilitate such drying.

In an alternative embodiment, the afore-mentioned gallery 1409 does notextend around the entire circumference of the housing, so as to seal theplenum 1406 on all sides. It can be a “partial” gallery or a combinationof one or more air knives (with negative or positive flow) positionedeither downstream or upstream of the spray head(s) and/or intermediateapplicator(s) in parallel to the axis of the drum and/or on the lateraledges of the spray heads and/or applicators in a direction perpendicularto the axis of the drum. A “partial” gallery on the exit side may, insome embodiments, serve as gas blower (e.g., cold or hot air)additionally or alternatively facilitating the drying of the particles,in which case the passages 1410 may be adapted to provide sufficientflow rate.

In the embodiment illustrated in FIG. 2, instead of being carried in afluid sprayed directly onto the donor surface 12, the suspendedparticles are applied by spray heads 1401 to an intermediate applicator1420. The applicator 1420 may be for example a sponge-like roller, ofwhich the axis is parallel to the axis of drum 10. The fluid andsuspended particles may be sprayed onto the applicator 1420 in themanner shown in FIG. 2, or if the applicator is porous, or constructedin manner similar to the “brushes” used in automatic car washes thathave loose fabric strips extending radially from a central axle, thenthe fluid may be introduced via the axle hub and allowed to escapethrough holes in the axle (not shown). The material of the roller or thefabric strip is to be “relatively soft”, selected so as to wipe theparticles on the surface, without affecting the integrity of the coatthereupon formed, in other words without scratching the layer ofparticles. The surface of the applicator, or of its bristles or stripes,may suitably comprise a closed-cell foam (such as such as closed cellpolyethylene, closed cell PVA or closed cell silicone); or a relativelysoft open-cell foam (such as a polyurethane foam); or a fabric, such ascotton, silk or ultra high molecular weight polyethylene (UHMWPE)fabric.

As the roller or brush 1420 rotates along its axis, it applies theparticles upon contact with donor surface 12 of drum 10. The outersurface of the applicator 1420 need not have the same linear velocity asthe donor surface and it can, for instance, be up to about ten-foldhigher. It may rotate in the same direction as drum 10 or incounter-direction. The applicator may be independently driven by a motor(not shown, in FIG. 2), or driven by drum 10 by gears, belts, friction,and the like.

The particle coating apparatus 14 may comprise more than one applicator1420 of particles, e.g., two or three applicators, as schematicallyillustrated in FIG. 3. In the figure, showing a partial view of acoating station 14 and of a donor surface 12 mounted on a drum 10, threeapplication stations 1430 a, 1430 b and 1430 c are illustrated. Eachsuch station, as detailed for 1430 a, may have in addition to itsapplicator 1420 a, its own supply of particles as applied by sprays 1402a, provided by spray heads 1401 a, the relevant fluid being delivered bysupply conduct 1405 a. Such applicator(s) may optionally provide someburnishing or flattening of the particles on the donor surface, or suchfunction, if desired, can be provided by a separate element, such asroller 40 described below.

The coating apparatus can also further comprise a cleaning roller. Acleaning roller can be similar in structure to an applicator roller,except that it would lack the supply of particles. A cleaning roller mayfor instance apply a liquid corresponding to the fluid carrier of theparticles, but depleted of the latter. In the example illustrated inFIG. 3, stations 1430 a and 1430 b may serve to apply particles, whilethe applicator of 1430 c may serve as cleaning roller. Alternatively,the cleaning roller, if present, may be positioned externally to thehousing of the particles applicator(s), optionally in a separate housingwith a distinct fluid supply and system for elimination and/orrecirculation.

A cleaning device, if present, can be continuously operated. Forinstance, a cleaning roller as above-exemplified may serve to removeparticles not in direct contact with the donor surface during any cycleof the surface in the coating station during operation of the printingsystem. Additionally, and alternatively, a cleaning device can be usedperiodically. Such a cleaning device may for instance be used formaintenance, and can serve to remove all particles from the entire donorsurface. Such complete regeneration of the donor surface to be free ofparticles can be done intermittently or periodically, for example at theend of a print job, or when changing the particles to be printed (e.g.,to a new batch or to a new type), or once a day, or once a week, or anyother desired frequency. Periodical cleaning devices, which may rely onchemical or physical treatment of the donor surface achieving fullparticle removal, can be located externally to the coating station. Theycan be operated for at least one cycle of the donor surface.

The Particles

The shape and composition of the coating particle will depend inpractice on the nature of the effect to be applied to the surface of thesubstrate 20. In a printing system seeking to achieve effects similar tofoil printing, the particles may conveniently be formed of a metallic ormetal-looking material. For printing of high quality, it is desirablefor the particles to be as fine as possible to minimize the intersticesbetween particles of the applied monolayer coating. The particle size isdependent upon the desired image resolution and for some applications aparticle size (e.g., a diameter or maximum long dimension) of 10 μm(micrometers) or possibly even more (i.e. having a larger size) mayprove adequate. The longest dimension of irregular platelets may evenreach 100 μm on average. However, for improved image quality, it ispreferred for the particle size to be a small fraction or a fraction ofa micrometer and more preferably a few tens or hundreds of nanometers.Commercially available flakes may have a thickness of about 60-900 nmand a representative planar dimension (e.g., mean diameter for nearround flakes or average “equivalent diameter” for platelets having lessregular plane projection, also characterized by shortest/longestdimensions) of about 1-5 μm, but flakes can also be prepared with athickness of as little as 15 nm, 20 nm, 25 nm, 30 nm, 40 nm, or 50 nmand a mean or equivalent diameter in the region of 100-1000 nm or500-800 nm. When metal-looking particles are used, it is believed thatover most of the practical size range, the smaller the particle size,the greater is the degree of shine that can be achieved and the closerone can approximate to a mirror-like finish when such particles havesubstantially the same orientation (e.g., when flake-like particles areto a large extent aligned with one another, so as to form a relativelyeven surface enhancing specular light reflection). However, the size ofthe particles need not be too small, since below a certain threshold,which typically depends on the chemical and/or physical nature of theparticles, the particles may display undesired edge effects renderingthem less suitable to the intended printing. Thus ideal sizedetermination, which may depend upon the intended visual effect as wellas other printing parameters (e.g., substrate and/or receptive layerroughness) or operating parameter of the printing system (e.g.,impression pressure or burnishing shear and like factors), may be doneempirically, by routine experimentation, by one of ordinary skill in theprinting art.

Particles of both pigmented non-thermoplastic polymers and metal-lookingmaterials may be used to achieve either a matte or a glossy look, andany intermediate look, once transferred to the printing substrate. Suchlook may, to some extent, be subsequently modified by additional steps(e.g., burnishing, varnishing, etc.)

Depending on their shape, which can be relatively regular or irregular,the particles may be characterized by their length, width, thickness,mean or equivalent diameter or any such representative measurement oftheir X-, Y- and Z-dimensions. Generally the dimensions of the particlesare assessed on planar projections of their shape (e.g., vertical and/orhorizontal projections). Typically such sizes are provided as average ofthe population of particles and can be determined by any technique knownin the art, such as microscopy and Dynamic Light Scattering (DLS). InDLS techniques the particles are approximated to spheres of equivalentbehavior and the size can be provided in term of hydrodynamic diameter.DLS also allows assessing the size distribution of a population. As usedherein, particles having a size of, for instance, 10 μm or less, have atleast one dimension smaller than 10 μm, and possibly two or even threedimensions, depending on shape. The particles are said to fulfill onaverage any desired size preference, if the D50 (up to 50% of thepopulation) is about the intended size; whereas a population ofparticles wherein the D90 is about the intended size implies a vastmajority of particles (up to 90% of the population) satisfy the same.

The particles may have, depending on shape, a variety of “characteristicdimensions”, such as a long dimension or a maximum long dimension, whichmay be averaged to characterize a population consisting of a pluralityof such particles, this representative value being termed Lavg.Particles can additionally be characterized by a short dimension or amaximum short dimension, the short dimension typically being thethickness of the particles for platelet shaped ones. This secondcharacteristic dimension can also be averaged to identify the relevantpopulation of particles, this representative value being termed Havg.

Particles suitable for the present printing system and method may havean average maximum long dimension Lavg of at most 800 micrometers, atmost 600 μm, at most 400 μm, at most 250 μm, at most 150 μm, at most 100μm, at most 80 μm, at most 60 μm, at most 40 μm, at most 25 μm, at most20 μm, at most 15 μm, at most 12 μm, at most 10 μm, at most 8 μm, atmost 6 μm at most 4 μm, at most 3 μm, at most 2 μm, at most 1.5 μm, atmost 1.2 μm, at most 1.0 μm, at most 0.8 μm, at most 0.7 μm, at most0.65 μm, or at most 0.6 μm. Additionally, the average maximum longdimension can be at least 0.04 micrometers, at least 0.05 μm, at least0.06 μm, at least 0.08 μm, at least 0.10 μm, at least 0.12 μm, at least0.15 μm, or at least 0.20 μm.

Particles suitable for the present printing system and method mayfurther have an average maximum thickness Havg which can be of at most1200 nm, at most 1000 nm, at most 800 nm, at most 600 nm, at most 500nm, at most 400 nm, at most 350 nm, at most 300 nm, at most 250 nm, atmost 200 nm, at most 175 nm, at most 150 nm, at most 125 nm, or at most100 nm. Additionally, the average maximum thickness can be at least 5nm, at least 7 nm, at least 10 nm, at least 15 nm, at least 20 nm, atleast 25 nm, at least 30 nm, at least 40 nm, or at least 50 nm.

Though not essential, the particles may preferably be uniformly shapedand/or within a symmetrical distribution relative to a median value ofthe population and/or within a relatively narrow size distribution.

A particle size distribution is said to be relatively narrow if at leastone of the two following conditions applies:

-   -   A) the difference between the hydrodynamic diameter of 90% of        the particles and the hydrodynamic diameter of 10% of the        particles is equal to or less than 150 nm, or equal to or less        than 100 nm, or even equal to or less than 50 nm, which can be        mathematically expressed by: (D90−D10)≤150 nm and so on; and/or    -   B) the ratio between a) the difference between the hydrodynamic        diameter of 90% of the particles and the hydrodynamic diameter        of 10% of the particles; and b) the hydrodynamic diameter of 50%        of the particles, is no more than 2.0, or no more than 1.5, or        even no more than 1.0, which can be mathematically expressed by:        (D90−D10)/D50≤2.0 and so on.

The particles may have any suitable aspect ratio, i.e., a dimensionlessratio between the smallest dimension of the particle and the equivalentdiameter in the largest plane orthogonal to the smallest dimension. Theequivalent diameter can be for instance the arithmetical average betweenthe longest and shortest dimensions of that largest orthogonal plane.Such dimensions are generally provided by the suppliers of suchparticles and can be assessed on a number of representative particles bymethods known in the art, such as microscopy, including in particular byscanning electron microscope SEM (preferably for the planar dimensions)and by focused ion beam FIB (preferably for the thickness and lengthdimensions). Particles having an almost spherical shape arecharacterized by an aspect ratio of approximately 1:1, whereasflake-like particles can have an aspect ratio (i.e. between the averageof the longest lengths of planar projections of the particles (maximumlong dimension) or of their mean or equivalent diameters, as the casemay be, and the average thickness (maximum short dimension) of theparticles) of 100:1 or more. Though not limiting, the particlesaccording to the present teachings can have an aspect ratio (or averageaspect ratio being defined by: ASPavg=Lavg/Havg) of about 100:1 or less,of about 75:1 or less, of about 50:1 or less, of about 25:1 or less, ofabout 10:1 or less, or even of about 2:1 or less. In some embodiments,the particles according to the present teachings may have an aspectratio (or average aspect ratio) of at least 2:1, at least 3:1, at least5:1, at least 10:1, at least 25:1, at least 40:1, or at least 70:1. Insome embodiments, the particles according to the present teachings mayhave an aspect ratio (or average aspect ratio) within a range of 2:1 to500:1, 4:1 to 500:1, 8:1 to 500:1, 20:1 to 500:1, 20:1 to 300:1, 20:1 to250:1, 20:1 to 200:1, or 20:1 to 100:1.

In these embodiments, the (overall or average) maximum long dimension,maximum short dimension and aspect ratio for a group of particles may bevolume-averaged, surface-area averaged, or number averaged.

In some embodiments, the aspect ratios of representative particles maybe estimated by SEM techniques and/or by SEM-FIB techniques, asdescribed in further detail herein.

While selecting a representative particle, or a group of representativeparticles that may accurately characterize the aspect ratio of thepopulation, it will be appreciated that a more statistical approach mayyet more accurately characterize the aspect ratio of particles withinthe population. Thus, in some embodiments of the present disclosure, theaspect ratio of the particles may be determined by analyzing, in itsentirety, a representative field of view of the image-capturinginstrument (e.g., SEM). Typically, the magnification is adjusted suchthat at least 5 particles, at least 10 particles, at least 20 particles,or at least 50 particles are disposed within a single field of view. Asabove, the (overall or average) aspect ratio for a group of particlesmay be volume-averaged, surface-area averaged, or number averaged.

As used herein in the specification and in the claims section thatfollows, the term “aspect ratio”, or “particular aspect ratio” refers tothe dimensionless ratio between the smallest dimension of the particleand the equivalent diameter in the largest plane orthogonal to thesmallest dimension.

As used herein in the specification and in the claims section thatfollows, the term “equivalent diameter” refers to the arithmeticalaverage between the longest and shortest dimensions of that largestorthogonal plane.

As used herein in the specification and in the claims section thatfollows, the term “average aspect ratio” or “overall aspect ratio”refers to the aspect ratio of a plurality of particles, each having aparticular aspect ratio.

In addition to their impact on the visual effect to be imparted, theparticles may have shapes and/or dimensions suitable to provide forsufficient contact area with the donor surface, and subsequently withthe desired substrate regions (e.g., on a receptive layer), at leastover a time period the visual effect is desired or until an overcoat isapplied.

Depending on their composition and/or on the processes they undergo(e.g., milling, recycling, burnishing, etc.), the particles can behydrophobic with different degrees, if any, of hydrophilicity. As thebalance between the hydrophobic and hydrophilic nature of the particlesmay shift with time, the process is expected to remain efficient if thehydrophobic nature of the particles predominates. Additionally, theparticles may be made of materials intrinsically hydrophilic, in whichcase they can be rendered hydrophobic by application of a particlecoating. Materials suitable for such a particle coating can have ahydrophilic end with affinity to the particle (e.g., a carboxylicfunction affine to a metal oxide) and a hydrophobic tail. In the presentdisclosure such particles, whether intrinsically hydrophobic or coatedto become hydrophobic or more hydrophobic, are said to be substantiallyhydrophobic.

The hydrophobicity of the particles may be a known property inherent totheir chemical composition. If needed, the degree of hydrophobicity orhydrophilicity can be assessed by measurement of the contact angle of adroplet of reference liquid (typically deionized water) on a sizeablesurface of the bulk material forming the particles or of their coat, asapplicable. Those of skill in the art will readily appreciate that acontact angle may be used to characterize a degree of hydrophilicity orhydrophobicity, according to standard techniques. A contact angle above90° may indicate a hydrophobic surface, while a contact angle below thisvalue may be indicative of a hydrophilic surface. Additionally,hydrophobicity may be assessed at the scale of the particles byintroducing a predetermined amount of the particles to deionized water.Hydrophobic particles will display a leafing behavior, migrating towardsthe air interface, while hydrophilic particles will exhibit anon-leafing pattern, allowing them to maintain a fairly randomdistribution in the water carrier. Such phase separation, or lackthereof, can be facilitated by the addition of a non-water miscible oilphase, in which case the hydrophobic particles migrate towards the oilphase, while hydrophilic particles tend to remain in the aqueous phase.Determination of the concentration of particles in the initial watersample and in the final separated phases, the phase separation beingtypically performed three times for a given sample, allows assessing thehydrophobic or hydrophilic behavior of the particles. Additional methodscan be used, such as surface adsorption assays using a known proportionof Rose Bengal dye per the amount of particles to be tested. The dyeadsorbs on hydrophobic surface of particles as a function of theirsurface area. The unbound dye remaining in the aqueous phase can bemeasured by spectrophotometry, providing an estimate of the bound amountcommensurate with the hydrophobicity of the particles. The relativehydrophobicity can be determined by calculating the Partition Quotientof the dye between the amount absorbed and the unbound amount.Similarly, Nile Blue dye can be used to determine the hydrophilicity ofthe particles surface. Additional methods are known and can be suitable.As used herein, the term “hydrophobic” and the like is used forparticles and materials that exhibit hydrophobicity according to atleast one (and preferably at least two or three) of the above-describedcharacterization methods.

In one embodiment, the particles are of aluminum and are coated with acarboxylic acid that renders the particles hydrophobic, reduces theirability to stick to one another and reduces their oxidation. Thehydrophobicity of such particles when coated with stearic acid wasestimated by measuring the contact angle formed by a droplet ofdeionized water, according to the method described in more details forthe donor surface. Such coated particles displayed a wetting angle of130.1+6°. However, as mentioned, particles having any wetting angleexceeding 90° can be suitable.

The particles can be carried by either a gaseous or a liquid fluid whenthey are sprayed onto the donor surface or upon the intermediateapplicator(s). When the particles are suspended in a liquid, in orderboth to reduce cost and minimize environmental pollution, it isdesirable for the liquid to be aqueous. In such a case, it is desirablefor the material used to form or coat the particles to be hydrophobic.Hydrophobic particles more readily separate from an aqueous carrier,facilitating their tendency to attach to and coat the donor surface.Such preferential affinity of the particles towards the donor surface ofthe coating device, rather than towards their carrier and towards oneanother, is deemed particularly advantageous. Blowing a gas stream overthe particle coating (which as mentioned can preferably be formed byhydrophobic particles on an hydrophobic surface) will both serve todislodge and/or entrain particles not in direct contact with the donorsurface and to at least partially dry the coating on the donor surface.

When applying to the substrate 20 an effect similar to foil imaging, theparticles may be, as mentioned, metallic or more generally metal-lookingand may be coated or uncoated. Because of the manner in which suchparticles are produced (commonly by milling), they tend to be flatplatelets and though not essential this enables highly reflectivecoatings of near mirror quality to be achieved when the particles havelight reflective surfaces and their planar dimension are substantiallyaligned with the surface of the substrate. Such particles lendthemselves to burnishing, which may be carried our either by the use ofhigh pressure during the spraying or by means of a burnishing roller,such as the optional roller 40 and counter roller 42 shown in FIG. 2.

In addition, or as an alternative, to burnishing the particle layerafter it has been transferred to the substrate, it is possible toburnish it while it is still on the donor surface 12. Thus, a burnishingroller or other wiping element may be positioned immediately downstreamor as part of the coating apparatus 14.

Burnishing may be carried out with a dry roller or with a wet roller(e.g., impregnated and/or washed with the fluid on which the particlesare suspended, for instance water). In the event that an intermediateapplicator is used, it cannot be ruled out that it may, in addition toapplying the particles to the donor surface also at least partly burnishthem. It is believed that during burnishing, the size of the particlesis reduced as compared to their original size upon initial injectioninto the coating apparatus, and that, alternatively and additionally,the burnished particles are oriented in a substantially parallel mannerwith respect to the donor surface.

The outer surface of the optional burnishing roller may rotate at alinear speed different than that of the donor surface of the drum and/orof the outer surface of an intermediate applicator, if present. It canrotate in the same or counter-direction relative to the drum.

The Particle Carrier

The particle carrier, that is to say the fluid within which theparticles are suspended, may be either a liquid or a gas. If liquid, thecarrier is preferably water based and if gaseous the carrier ispreferably air. The particles may be lyophobic (i.e., having noaffinity) with respect to their carrier, for instance may behydrophobic, while the carrier is an aqueous liquid. Such may result inparticles being partly dispersed in the liquid, and partly phaseseparated (all types of such mixtures of materials of same or differentphases being herein encompassed by the term “suspended”). In addition tothe particles, the carrier may comprise any additive known in the art ofparticle formulation, such as dispersants, surfactants, water-misciblesolvents, co-solvents, stabilizers, preservatives, viscosity modifiers,pH modifiers, and the like. All such additives and their typicalconcentrations are known to persons skilled in the art of dispersionsand need not be further detailed herein. Additives (or mixtures thereof)not affecting the hydrophobicity of the particles and of the donorsurface are preferred. Such agents, in particular the dispersing agents,may assist in maintaining or increasing the stability of the suspendedparticles in the liquid (including in phase separated form, if desired).The liquid carrier may also comprise excess of unbound material servingas particle coat, if desired when applicable. Any such additive and mixthereof, preferably do not affect the overall inertness of the liquidcarrier towards the donor surface (e.g., avoiding or reducing anydeleterious swelling of the surface that would prevent proper coating byattachment of the particles).

A liquid carrier is said to be aqueous if it contains at least 80 wt. %water (i.e., 80% by weight of the total composition), or at least 85 wt.%, or at least 90 wt. %, or at least even 95 wt. % water. It is to beunderstood that though final work aqueous compositions may predominantlycontain water, as previously mentioned, it is possible to prepareintermediate aqueous compositions containing a higher amount of solidparticles (and additives if any) and lower amount of water. Suchintermediate compositions may serve as concentrates, which can bediluted to desired working concentrations when needed, but stored and/orshipped in smaller volumes. A concentrate may for instance comprise asmuch as about 80 wt. % of solids and about 20 wt. % of a water miscibleco-solvent, the water being added during dilution of the concentrate.

The Donor Surface

The donor surface 12 in some embodiments is a hydrophobic surface, madetypically of an elastomer that can be tailored to have properties asherein disclosed, generally prepared from a silicone-based material.Poly(dimethylsiloxane) polymers, which are silicone-based, have beenfound suitable. In one embodiment, a fluid curable composition wasformulated by combining three silicone-based polymers: avinyl-terminated polydimethylsiloxane 5000 cSt (DMS V35, Gelest, CAS No.68083-19-2) in an amount of about 44.8% by weight of the totalcomposition (wt. %), a vinyl functional polydimethyl siloxane containingboth terminal and pendant vinyl groups (Polymer XP RV 5000, EvonikHanse, CAS No. 68083-18-1) in an amount of about 19.2 wt. %, and abranched structure vinyl functional polydimethyl siloxane (VQMResin-146, Gelest, CAS No. 68584-83-8) in an amount of about 25.6 wt. %.To the mixture of the vinyl functional polydimethyl siloxanes wereadded: a platinum catalyst, such as a platinumdivinyltetramethyldisiloxane complex (SIP 6831.2, Gelest, CAS No.68478-92-2) in an amount of about 0.1 wt. %, an inhibitor to bettercontrol curing conditions, Inhibitor 600 of Evonik Hanse, in an amountof about 2.6 wt. %, and finally a reactive cross-linker, such as amethyl-hydrosiloxane-dimethylsiloxane copolymer (HMS 301, Gelest, CASNo. 68037-59-2) in an amount of about 7.7 wt. %, which initiates theaddition curing. This addition curable composition was shortlythereafter applied with a smooth leveling knife upon the support of thedonor surface (e.g., an epoxy sleeve mountable on drum 10), such supportbeing optionally treated (e.g., by corona or with a priming substance)to further the adherence of the donor surface material to its support.The applied fluid was cured for two hours at 100-120° C. in a ventilatedoven so as to form a donor surface.

The hydrophobicity is to enable the particles exposed to selectivestripping by the tacky film created on the receptive layer bearingsubstrate to transfer cleanly to the substrate without splitting.

The donor surface should be hydrophobic, that is to say the wettingangle with the aqueous carrier of the particles should exceed 90°. Thewetting angle is the angle formed by the meniscus at theliquid/air/solid interface and if it exceeds 90°, the water tends tobead and does not wet, and therefore adhere, to the surface. The wettingangle or equilibrium contact angle Θ₀, which is comprised between andcan be calculated from the receding (minimal) contact angle Θ_(R) andthe advancing (maximal) contact angle Θ_(A), can be assessed at a giventemperature and pressure of relevance to the operational conditions ofthe process. It is conventionally measured with a goniometer or a dropshape analyzer through a drop of liquid having a volume of 5 μl, wherethe liquid-vapor interface meets the solid polymeric surface, at ambienttemperature (circa 23° C.) and pressure (circa 100 kPa). Contact anglemeasurements can for instance be performed with a Contact Angleanalyzer—Krüss™ “Easy Drop” FM40Mk2 using distilled water as referenceliquid.

Such measurements were performed on a sample of donor surface preparedas above described, the sample having a size of 2 cm×2 cm. The resultswere analyzed using “Drop shape analysis” program, circle computermethod, the advancing contact angle Θ_(A) of the above-described donorsurface was found to be 101.7°±0.8° and the receding contact angle Θ_(R)was found to be 99.9°±3.1°. Typically, donor surfaces prepared by thismethod had contact angles in the range of about 95° to about 115°,generally not exceeding 110°.

This hydrophobicity may be an inherent property of the polymer formingthe donor surface or may be enhanced by inclusion of hydrophobicityadditives in the polymer composition. Additives that may promote thehydrophobicity of a polymeric composition may be, for example, oils(e.g., synthetic, natural, plant or mineral oils), waxes, plasticizersand silicone additives. Such hydrophobicity additives can be compatiblewith any polymeric material, as long as their respective chemical natureor amounts do not prevent proper formation of the donor surface, and forinstance would not impair adequate curing of the polymeric material.

The roughness or finish of the donor surface will be replicated in theprinted metallised surface. Therefore if a mirror finish or highlyglossy appearance is required, the donor surface would need to besmoother than if a matte or satin look is desired. These visual effectscan also be derived from the roughness of the printing substrate and/orof the receptive layer.

The donor surface 12 may have any Shore hardness suitable to provide astrong bond to the particles when they are applied using the coatingapparatus 14, the bond being stronger than the tendency of the particlesto adhere to one another. The hardness of the silicone-based surface mayvary and for instance depend on the thickness of the donor surfaceand/or the particles intended to be bond. It is believed that forrelatively thin donor surfaces (e.g., 100 μm or less), thesilicone-based material may have a medium to low hardness; whereas forrelatively thick donor surfaces (e.g., up to about 1 mm), thesilicone-based material may have a relatively high hardness.Additionally, larger particles may typically benefit from a donorsurface having a lower hardness than necessary to accommodate relativelysmaller particles. In some embodiments, a relatively high hardnessbetween about 60 Shore A and about 80 Shore A is suitable for the donorsurface. In other embodiments, a medium-low hardness of less than 60,50, 40, 30 or even 20 Shore A is satisfactory.

The donor surface 12 in the drawings is the outer surface of a drum 10but this is not essential as it may alternatively be the surface of anendless transfer member having the form of a belt guided over guiderollers and maintained under an appropriate tension at least while it ispassing through the coating apparatus. Additional architectures mayallow the donor surface 12 and the coating station 14 to be in relativemovement one with the other. For instance, the donor surface may form amovable plan which can repeatedly pass beneath a static coating station,or form a static plan, the coating station repeatedly moving from oneedge of the plan to the other so as to entirely cover the donor surfacewith particles. Conceivably, both the donor surface and the coatingstation may be moving with respect to one another and with respect to astatic point in space so as to reduce the time it may take to achieveentire coating of the donor surface with the particles dispensed by thecoating station. All such forms of donor surfaces can be said to bemovable (e.g., rotatably, cyclically, endlessly, repeatedly movable orthe like) with respect to the coating station where any such passingdonor surface can be coated with particles (or replenished withparticles in exposed regions).

The donor surface may additionally address practical or particularconsiderations resulting from the specific architecture of the printingsystem. For instance, it can be flexible enough to be mounted on a drum,have sufficient abrasion resistance, be inert to the particles and/orfluids being employed, and/or be resistant to any operating condition ofrelevance (e.g., pressure, heat, tension, etc.). Fulfilling any suchproperty tends to favorably increase the life-span of the donor surface.

The donor surface, whether formed as a sleeve over a drum or a belt overguide rollers, may further comprise, on the side opposite the particlereceiving outer layer, a body, which together with the donor surface maybe referred to as a transfer member. The body may comprise differentlayers each providing to the overall transfer member one or more desiredproperty selected, for instance, from mechanical resistivity, thermalconductivity, compressibility (e.g., to improve “macroscopic” contactbetween the donor surface and the impression cylinder), conformability(e.g., to improve “microscopic” contact between the donor surface andthe printing substrate on the impression cylinder) and any suchcharacteristic readily understood by persons skilled in the art ofprinting transfer members.

The Treating Station

As mentioned, numerous ways of applying a receptive layer (e.g., anadhesive or activatable adhesive) pattern to a printing substrate areknown, especially in conventional non-digital printing systems asdiscussed in relation with the possible alternative analog treatingstations 36 schematically illustrated in FIG. 2. The imaging system 16schematically illustrated in FIG. 1 provides one way of selecting theregions on the substrate where the particle coating applied to the donorsurface 12 that will transfer to the substrate 20 at the impressionstation. Such an imaging system is required in the implementation of adigital treating station for a digital printing system.

An exemplary imaging system 16 may comprise a support 1601 carrying anarray of laser sources such as VCSEL (Vertical Cavity Surface EmittingLaser) chips 1602 that are optionally arranged in pair(s) of rows inpositions that are accurately predetermined relative to one another(e.g., in a staggered manner providing laser sources suitable to targetpoints along the entire width of the substrate). The support 1601 may befluid cooled to cope with the significant heat that may be generated bythe chips. Laser beams emitted by the chips 1602 are focused by lenses1603 constructed as two or more corresponding rows of GRIN(Gradient-Index) rod lenses (each chip 1602, and all laser elementsthereupon, being associated with a corresponding focusing lens 1603).Signals supplied to the chips for the activation of one or more laserelement are synchronized with the movement of the substrate 20 in thedirection of the illustrated arrow (i.e. from the treating or imagingstation towards the impression station) by a transport system (not shownin FIG. 1). The effect of the irradiation of each pixel by a laser beamis to convert an inactive receptive layer on the substrate 20 at thatpixel into a tacky state (i.e. an active receptive layer) so thatparticles coating the donor surface 12 may later transfer and adherethereto. In other words, such irradiation mediated activation of thereceptive layer provides on the substrate selected areas 24 having moreaffinity towards the particles than the particles have with the donorsurface, the activated areas thus being able to selectively detachparticles from the donor surface 12.

If used for color printing, the systems shown in FIGS. 1 and 2 can onlyprint in one color but multicolor printing can be achieved by passingthe same substrate successively through multiple towers that aresynchronized with one another and each printing a different color.Alternatively, and additionally, different colors can be obtained byapplying a colored transparent overcoat (or a partial foreground image)above particles having a sufficiently light shade. For instance, a“gold” look can be achieved by overprinting a yellow-orange tint over“silver” looking aluminum particles.

The Substrate

The printing system shown in the drawing is not restricted to anyparticular type of substrate, as long as the particles have higheraffinity towards the donor surface than to the bare substrate (i.e., inareas lacking a suitable receptive layer). The substrate may beindividual sheets of paper or card or it may have the form of acontinuous web. The substrate can also be made of a fabric or ofleather. Because of the manner in which the particles are applied to thesubstrate, the particles tend to reside on the surface of the substrate.This allows printing of high quality to be achieved on paper ofindifferent quality. Furthermore, the material of the substrate need notbe fibrous and may instead be any type of surface, for example aplastics film or a rigid board. As previously explained, the substratemay also have any desired roughness adapted to the desired look, thoughsuch intended effect can also be modulated at the level of the receptivelayer.

It should be recalled that some printing substrates may be supplied incoated or uncoated forms, or be otherwise pre-treated to facilitatetheir intended use. For instance, a substrate may be coated with apriming material that may enhance the later adhesion of a receptivelayer to the substrate, or enable any other like step the substrate maybe subjected to. In the present specification, the term “substrate” isto be understood in its broadest sense, irrespective of form, materialand coating(s) or lack thereof, as a physical support to an image to beor having been printed, in particular able to bear the particles to betransferred thereupon.

The Impression Station

The illustrated impression station 18 comprises only a smooth impressioncylinder 22 that is pressed against the drum 10 and its outer donorsurface 12. The impression cylinder 22 may form part of a substratetransport system, in which case it may be equipped with grippers forengaging the leading edge of individual substrate sheets. Alternatively,the impression cylinder may bear a shape serving to further emboss theprinting substrate to which the particles are being transferred.

As mentioned, a printing system according to present teachings mayinclude more than one impression station. Separate impression stations,typically allowing the deposition of different compositions upon asubstrate, such as the transfer of different particles or the printingof different colors or of different visual effects with a same color,may each include a different impression cylinder. However this needs notnecessarily be the case. For instance, two or more treating stations(whether digital as illustrated by station 16 or analog as illustratedby station 36) can each have their respective downstream coatingstation/donor surface, and be radially positioned to face a singleimpression cylinder. This is schematically illustrated in FIG. 4 whichexemplifies a case of three coating stations 14 a, 14 b and 14 c, eachpreceded on its upstream side by a respective treating station 46 a, 46b and 46 c, the treating of substrate 20 or of a receptive layerthereupon being achieved by any suitable mean, as previously exemplifiedin a non-limitative manner with stations 16 and 36. The nips betweendonor surfaces 12 a, 12 b and 12 c, and impression cylinder 22 form theradially disposed impression stations 18 a, 18 b and 18 c. As previouslyexplained, though FIG. 4 illustrates a plurality of impressions stationsaccording to the present teachings, the printing system of the inventionmay alternatively and additionally include conventional impressionstation(s). Such stations may serve to print a background image to theselected regions to be coated with particles, or a foreground imagebeing printed after the particles are transferred to the substrate, orboth.

Furthermore, a printing system, even if mono-color, may include aperfecting system allowing double-sided printing. In some cases,perfecting can be addressed at the level of the substrate transportsystem, which may for example revert a substrate to a side not yetprinted on and reefed the unprinted side of the substrate to the sametreating and impressions stations having served to print the first side.In other cases, perfecting can be addressed by including two separateimpression stations (and their respective upstream or downstreamstations), each impression station enabling printing on a different sideof the same substrate.

Exemplary Print-Outs

FIG. 5A to 5D show pictures of printing substrates as used and obtainedaccording to the present teachings. The substrates were printed using aprinting system as schematically illustrated in FIG. 2 withmodifications as follows.

Briefly, the printing substrate was a web of either a synthetic paper(biaxially oriented polypropylene film (BOPP) White Matt P25 Synthetic54Glassine Liner 60 gsm, Nirotech Adhesives & Coating Technologies,Israel) or a polypropylene plastic foil upon which a lacquer (Wessco®3501 UV-varnish of Schmid Rhyner AG, Switzerland), was applied byflexographic printing at a linear velocity of 30 m/min to form, uponsufficient curing, a receptive layer 26. The thickness of the resultinglayer was about 3.6-4.2 μm, as was determined by Laser ConfocalMicroscopy (Olympus, LEXT). The particles supplied to the coatingstation, to be dispensed upon the donor surface so as to substantiallyform a monolayer, were aluminum flakes (Aluminum powder 6150 supplied byQuanzhou Manfong Metal Powder Co., China, CAS No. 7429-90-5) having aroughly platelet shape with an average diameter of about 4 μm and anaverage thickness of about 70 nm. The particles were fed at a weightconcentration of about 3 wt. % in water and sprayed upon a rollingcylindrical sponge serving as intermediate applicator 1420. The donorsurface 12 was made of silicone-based polymers consisting of vinylfunctionalized polydimethylsiloxane (PDMS), the addition curableformulation and preparation of which were detailed above. The printingsubstrate, including the patterns of the receptive layer appliedin-line, was fed to the inventive printing system at ambienttemperature, at a linear velocity of 30 m/min, and the force applied atthe nip of the impression station was about 12 kg-f/cm.

FIG. 5A shows a picture of the substrate before its feeding to theimpression station, the darker patterns corresponding to the receptivelayer as applied by flexographic printing as explained above. Forenhanced visibility of the receptive layer, the BOPP white syntheticpaper substrate was pre-printed with a black background image prior tothe application of the receptive layer pattern. FIG. 5B shows a pictureof a white BOPP paper substrate after its exit from the impressionstation, following its contacting with the aluminum particles coated onthe donor surface, the darker patterns corresponding to the transferredparticles. FIGS. 5C and 5D show similar post-impression pictures withcontrasted metallised patterns, the substrate used in FIG. 5C being atransparent plastic foil (placed on a white background for the sake ofthe picture) and the substrate used in FIG. 5D being the black papersubstrate of FIG. 5A.

FIGS. 6A to 6D are magnified views of a section of FIGS. 5A to 5D,respectively. As can be seen, the receptive layer patterned on thesubstrate suitably detached at the impression station the aluminumparticles from the donor surface, so as to provide a corresponding metalprinted image downstream of the impression station. Such images were notfurther processed in any way (e.g., no burnishing, no varnishing, etc.).It is further noted that the transfer left the corresponding regions onthe donor surface 12 exposed (not shown), such regions being replenishedwith new particles upon completion of a subsequent cycle at the coatingstation.

Additional examples were printed using an alternative treating stationin a printing system as schematically illustrated in FIG. 2 withmodifications as follows. Briefly, the printing substrate was aphotographic paper (HP, USA) upon which a lacquer (UV Screen TactileVarnish, Cat. No. UVD0-1200-408N, Flint Group, Germany) was applied toform desired image patterns (e.g., including text and/or illustrations).The lacquer was applied by rotary silkscreen printing at a linearvelocity of 20 m/min, the screen having an open surface of 36% and amesh size of 165 μm. The layer formed on the substrate self-leveledwhile being transported to a curing station (e.g., for about 10 secondsor less). The lacquer coated and patterned substrate was carried out bya web substrate transport system comprising a unwinding roller supplyingbare substrate, a winding roller collecting the substrate including thedesired patterns of receptive layer, and intermediate rollers andsupport frames setting the path being traveled by the web substrate fromits entry feeding side to its delivery side. The curing station,disposed downstream of the treating station (where the lacquer wasapplied) and upstream of the delivery winding roller, included UV lamps,so as to partially cure the UV curable lacquer. The receptive layer maypreferably be cured to be sufficiently dry to the touch to permit thewinding of the substrate in a manner that would not be deleterious tothe receptive layer thereupon applied. Additionally, the receptive layertypically needs to remain sufficiently uncured so as to have enoughaffinity to the particles during printing (when contacting the particlesat the impression station 18). Once sufficiently dried, in the presentexample by partial curing, the receptive layer formed the desiredpatterns for subsequent application of the particles. The receptivelayer so formed had a thickness of about 52-65 μm above the surface ofthe substrate, as was determined by Laser Confocal Microscopy (Olympus,LEXT).

The above preparation of the substrate was performed off-line and thesubstrate was fed to the impression station of a printing systemaccording to present teachings, using a standard substrate transportsystem, similar to that previously described. For enhanced visibility ofthe receptive layer, the paper substrate was pre-printed with a blackbackground image prior to the application of the receptive layerpattern.

The printing substrate, including the patterns to become coated byparticles during impression, the affinity of the selective patterns tothe particles being higher than the affinity of the particles to thedonor surface, was fed at a linear velocity of 0.2 m/sec, though thesystem may be operating at any other suitable velocity (e.g., often upto 2 m/sec, but even up to 15 m/sec or more). The force at the nip ofthe impression station 18, between the donor surface 12 and theimpression cylinder 22 was of about 8 kg-f/cm and printing was performedat ambient temperature (circa 23° C.) without any further heating,neither at the nip nor upstream to the nip. Such operating conditionsare not to be construed as limiting.

The particles (same as previously described) were supplied to thecoating station at a weight concentration of about 0.1 wt. % in water toform a monolayer on a donor surface 12 made of PDMS, the additioncurable formulation and preparation of which were detailed above.

Results (not shown) were similar to those depicted in FIGS. 5A, 5D, 6Aand 6 D. Namely the pre-printed substrate 20 before its feeding to theimpression station, displayed a pattern darker than backgroundsubstrate, the pattern being formed from the material due to adhere tothe particles upon impression (i.e., the dried lacquer forming thereceptive layer 26). The same substrate after its exit from theimpression station 18, following its contacting with the outer surfaceof rotating drum 10 and transfer of the aluminum particles that werepreviously coated on the donor surface 12, displayed a metallisedversion of the pattern. This further demonstrates that the receptivelayer patterned on the substrate suitably detached at the impressionstation the aluminum particles from the donor surface, so as to providea metal printed image having a corresponding pattern downstream of theimpression station.

The speed at which a substrate 20 is transported along the variousstations at which it is processed and/or the distance between subsequentsuch stations can be used to modulate the duration of each step, alsoreferred to as the “residence time”, even though the substrate istypically in motion. For instance, the residence time at the treatingstation can affect the level of activation of selected regions or thethickness of the applied receptive layer 26 (which depending onviscosity of the constituting substance and its method of deposition canbe between a few and hundreds of micrometers). The receptive layerthickness that can be obtained by silkscreen printing application istypically between 50 and 500 μm, and more typically, at most 200 μm. Itcan be modified to provide an “embossing” look, if some distance betweenthe top of the receptive layer (subsequently the layer of particles) andthe substrate is desired. When using flexographic printing, a thinnerreceptive layer can be formed, having a thickness typically between 1 μmand 50 μm, and more typically, at most 15 μm.

It is believed that the residence time between the application of asubstance due to form the receptive layer 26 and its setting for asubsequent step (e.g., drying, curing, contacting of particles, etc.)may affect the topography of the outer surface of the receptive layer.For instance, for glossy effects, given sufficient time the just-appliedcoating may level on the surface of the substrate to form a receptivelayer having a substantially uniform thickness and/or having arelatively smooth outer surface. In such case, it may be preferable forthe substance forming the receptive layer to have time to degas (i.e.,reducing or eliminating the air bubbles that may be “entrapped” in thereceptive layer) to further improve the topographical properties of thereceptive layer outer surface so as to improve conformity to theparticles and/or to improve the transfer of the particles to theprinting substrate as a uniformly oriented mosaic of particles. Theresidence time at and following each station depends on the desiredprinting effect and on the materials being used in the process (e.g.,type of substrate, receptive layer and particles). It will beappreciated that such process adjustments are known to persons skilledin the art of printing.

Magnified Views of Metallised Print-Outs

Magnified views of print-outs obtained by known technologies and by thepresent invention were captured by confocal microscopy, in a mannerpreviously explained in relation with the assessment of the percentageof an area being covered by particles. The print-outs according to thepresent teachings were obtained by applying a receptive layer viasilkscreen printing on a printing substrate made of paper. The donorsurface and aluminum particles were as previously described, theparticles being at a weight concentration of 3 wt. % of the aqueouscomposition. The printing substrate, including the patterns of receptivelayer due to become coated by particles during impression, was fed atambient temperature, at a linear velocity of 0.5 m/sec, and the forceapplied at the nip of the impression station was about 12 kg-f/cm.

Representative magnified micrographs, all images being with the samemagnification, are shown in FIGS. 7A to 7F. Panels A to D of the figuredisplay images showing top views of conventionally metallised print-outsas obtained by: (A) Foil stamping (hot and cold techniques resulting ina substantially similar appearance); (B) Offset printing; (C) Gravureprinting; and (D) Flexography. FIG. 7E shows a similarly magnified viewof a print-out according to an embodiment of the present disclosure,whereas, for comparison, panel (F) displays a monolayer of particles asformed on the donor surface of the coating station, before beingtransferred to a printing substrate so as to form an image as magnifiedin panel (E). The scale bar in the lower left angle of all imagescorresponds to 40 μm.

As can be seen in panel (A), foil printing, whether hot (as shown) orcold, expectedly resulted in a continuous film of metal fully coveringall of the captured area. The particulate layers conventionally obtainedand illustrated in panels (B) to (D), are typically uneven, at least inone of the following aspects: a) the layer comprise stacks ofoverlapping particles; b) the stacks are randomly distributed, possiblyas a result of the limitations of each conventional printing technology;c) the thickness of the layer is irregular in particular in presence ofrandomly distributed stacks; and/or d) the voids between neighboringparticles are randomly distributed, possibly as a result of thelimitations of each conventional printing technology. It is observedthat the general appearance of the conventional print-outs from whichthe images of panels (B) to (D) were taken can be broadly described hasbeing hazier than the relatively more glossy print-out according to thepresent teachings. It is interesting to note that even the layer ofparticles formed on the donor surface can be relatively more glossy thanconventional print outs. This further suggests that particles asconventionally applied to metallise a surface are of irregularorientations, the amount of particles possibly parallel with the surfaceof the substrate being insufficient to provide enough light reflectionfor a glossy effect. In other words, the “reflective potential” ofsubstantially parallel particles is diminished or counterbalanced by the“scattering” effect of the particles having “non-parallel” randomorientations. In contrast with such comparative technologies relying onprinting of particles, the present method enables a more evenorientation of the particles, the particles being predominantly parallelto the substrate, as supported by the relatively high gloss andconfirmed by Atomic Force Microscopy (AFM) and FIB measurementsperformed on cross-sections of resulting print-outs.

It should be noted that, in contrast with metal inks used inconventional printing technologies, compositions of metal lookingparticles suitable for the present printing method need not to comprisea binder (e.g., a polymeric binder). Thus, the present monolayers aredevoid or substantially devoid of a binder, whereas metal ink imagescreated using such conventional necessarily binder-containing metal inkstypically appear on the printed substrate as a continuous film of binder“bridging” between adjacent particles. As conventionally used binderstypically envelop particles of the art in all three dimensions, theresulting print construction generally appears as a film of bindersurrounding randomly formed arrangements of particles, generallyentrapping the strata of particles and over-coating them. This isillustrated in FIGS. 9B and 9C, discussed herein-below.

The gloss of the metallised surface of printed samples can be measuredby any suitable instrument. In the present examples, it was measuredusing a Haze-gloss Reflectometer (BYK, Cat. No. AG-4601), theilluminator projecting the incident light and the detector measuring thereflected light at angles of 20° perpendicular to the surface, theilluminator and detector thus having 40° arc distance from one another.All samples tested were printed on paper substrate, having a size of 4cm×2 cm, the metallised samples corresponding to conventionaltechnologies being obtained from commercial printers. For eachtechnology, at least three randomly received samples were tested andtheir gloss values averaged. While it cannot be ruled out that eachconventional print-metallisation technology can yield higher results,the following gloss values are deemed representative and provided insupport of the even orientation of particles as applied and printedaccording to the present teachings.

Metallised surfaces printed as herein disclosed (using a silkscreenapplicator) displayed an average gloss of 426 Gloss Units (GU). Forcomparison, five foil-printed samples displayed an average gloss of 489GU; four offset-printed samples had an average gloss of about 22 GU;three gravure-printed samples had an average gloss of about 63 GU; andthree flexography-printed samples had an average gloss of about 55 GU.Therefore, the present technology provides with a monolayer of particlesa gloss comparable to foil printing, wherein the continuous film ofmetal is typically substantially parallel to the substrate surface. Whencompared to conventional technologies relying on individual particles,it can be seen that the present disclosure enables a significantlyhigher gloss corresponding to approximately 6.8-fold gravure-generatedgloss, about 7.7 fold flexography-generated gloss and about 19-foldoffset-generated gloss.

Normalizing such gloss values to the characteristic dimensions of theparticles or films involved in each process can provide an additionalmeasure of the outstanding outcome of the present printing method. AGloss Per Size (GPS) parameter, provided in Gloss Units per micrometer,is herein defined as the gloss of a printed sample divided by acharacteristic planar dimension of the gloss generating particle orfilm. The gloss can be measured as above-described and thecharacteristic dimension of the reflective surface of relevance to eachprinting technology or printed sample can be measured by confocalmicroscopy. Typically, such dimension is the average diameter or othercharacteristic dimension of at least twenty distinct particles deemedrepresentative of the population of particles sampled on the printedsubstrate being tested. In existing printing technologies, it istypically believed that small flakes, of less than 10 μm or 5 μm, are tobe avoided, particles having a distinct metallic character requiring anequivalent diameter of about 30 μm or more. Such small flakes,especially if smaller than 5 μm, are expected to cause a significantedge scattering effect, reducing the metallic brilliance and the glossof a printed construction. It is also believed that smaller particlesmay have a lower tendency than larger particle to adopt a parallelconfiguration of the flakes, such alignment when parallel to the surfaceof the printing substrate also contributing to gloss.

Such analysis was applied to the above gloss results, taking intoaccount measured average characteristic dimensions of about 2 μm for theparticles of the inventive printed constructions, as compared to about 5μm for the particles sampled in the offset printed construction, about10 μm for the particles sampled in the flexographic and gravure printedconstructions, and an infinite number arbitrarily set to 1000 μm for thecontinuous layer of the foil printed constructions. The GPS calculatedfor the printed constructions obtained by the method herein disclosedwas of about 230 GU/μm. The GPS calculated for the known technologies asassessed on the available samples did not exceed 10 GU/μm. Still itcannot be ruled out that such technologies could yield printedconstructions having a higher gloss and/or being formed from particleshaving a smaller characteristic dimension, resulting in a GPS of up to20 GU/μm, or up to 30 GU/μm, or up to 40 GU/μm, or even up to 50 GU/μm.Even then, it is clear that the GPS of the printed constructionsresulting from the method herein disclosed is significantly higher. Itis believed that the printed constructions according to the presentteachings can have, when using particles having a light reflectivesurface, a GPS of at least 100 GU/μm, or at least 150 GU/μm, or at least200 GU/μm, or at least 300 GU/μm, or at least 400 GU/μm, or even atleast 500 GU/μm. It is understood that such parameter is of relevanceonly if gloss is a desired quality of the printed constructions. Whenthe particles used in the present printing system and/or method are notintended to confer gloss to the printed construction, the GPS of theresulting printed constructions can be below 100 GU/μm.

Alternative Substrate Treating

A similar printing experiment was performed in which the receptive layerwas applied to a synthetic paper substrate (biaxially orientedpolypropylene film (BOPP) White Matt P25 Synthetic54 Glassine Liner 60gsm, Nirotech Adhesives & Coating Technologies, Israel) by flexographicprinting. Wessco® 3501 UV-varnish of Schmid Rhyner AG, Switzerland, wasapplied to the substrate 20 to form, upon sufficient curing, thereceptive layer 26. The thickness of the resulting layer was about3.6-4.2 μm. The appearance of the metallised images (coated withparticles of aluminum as above described) was as illustrated in FIGS. 5Ato 6D and comparable (data not shown) to images obtained withsilkscreen-applied receptive layers. Additionally, the layers ofparticles formed on such treated substrate were similar (data not shown)to those previously observed on magnified views of images obtained withsilkscreen-applied receptive layers (see FIG. 7E, for reference).

The percent area coverage or optical surface coverage was assessed aspreviously described. Briefly, samples were similarly prepared byflexographic printing of the receptive layer on a transparent plasticfoil of BOPP, images of metallised areas (i.e., formed on the layer)were captured by Optical Microscope (Olympus BX61 U-LH100-3) at amagnification of ×50 and analyzed in transmission mode. The results ofthree samples (each being an average of three images) were 81.3%, 84.9%and 86.4%.

Additional parameters were measured to compare a receptive layer asapplied by silkscreen printing with a receptive layer as applied byflexographic printing, these two techniques being non-limiting examplesof methods of treating a substrate for the present technology. Theroughness of an area of the top surface of each layer (before theircoating with particles) was measured using Laser Confocal Microscopy.The area roughness of the paper printing substrate upon which they wereapplied was measured as a reference. The substrate had an averagebaseline area roughness R_(a) of 0.61 μm. When the receptive layer wasapplied on this substrate by silkscreen printing (layer thickness ofabout 52-65 μm), the area roughness R_(a) of the top surface of thereceptive layer was about 0.46 μm. When the receptive layer was appliedon this substrate by flexographic printing (layer thickness of about3.6-4.2 μm), the area roughness R_(a) of the uppermost surface of theadhesive layer was 0.7 μm. Though not essential to the presenttechnology, as depending on the desired printing effect, it is believedthat receptive layers having a relatively low roughness (e.g., R_(a)≤2μm or even R_(a)≤1 μm) favor a more uniform orientation of theparticles, hence possibly a glossier appearance. It is also believedthat a thicker receptive layer, in particular given sufficient time toproperly level on the substrate and/or degas, may “absorb” and reducepart of the intrinsic roughness of the substrate, yielding an uppermostsurface having a lower roughness than a relatively thin receptive layerwhich follows more strictly the contour of the substrate surface. It isexpected that the contribution of the thickness of the receptive layerto the visible printing effect decreases for substrates being smooth perse (e.g., printing substrates made of plastic materials).

The various types of particle layers, which can be obtained by theprinting method described hereinabove, are schematically illustrated inthe cross-section along the x-y plane presented in FIGS. 8A and 8B.While particles 802, having an outer surface 804, are illustrated ashaving an elongated cross-sectional shape (e.g., corresponding to aplatelet like particle), this should not be construed as limiting.Particles 802 are positioned on top of a receptive layer 26, itselfselectively applied upon the image-receiving surface 80 of a printingsubstrate 20, such arrangement resulting in a printed construction 800having a monolayer 810 of particles. As previously explained, the outersurfaces 804 of particles 802 can be hydrophobic.

Referring to FIG. 8A, several particles are shown to be partiallyoverlapping, see section A, such overlap yielding an overall particlelayer thickness denoted as T. In section B, the particles areillustrated as being contiguous, whereas section C points to a gapbetween neighboring particles that is discernible from a directiongenerally orthogonal to the broad face of printing substrate 20. Insection D, a particle 806 is shown as having no contact with thereceptive layer, as appearing in the present x-y-cross section. However,such an overlapping particle may be positioned over the particlescontacting the underneath layer such that it could conceivably contactthe receptive layer at another point (not shown) along the z-direction.In section E, a particle 808 is shown as being overlapped by more thanone adjacent particle.

FIG. 8B illustrates an alternative embodiment, wherein the monolayer 810of particles is further coated with an overcoat 820. Though not shown inthe present illustration, it is believed that tiny air bubbles may beentrapped at or near the interface between the receptive layer 26 (andthe monolayer 810 of particles disposed thereupon) and the subsequentovercoat 820. Such phenomenon may facilitate the visualization of theboundary between such layers as can be assessed by FIB-SEM techniques orany other robust method.

It should be noted that while monolayer 810 is illustrated in FIGS. 8Aand 8B as being formed on top of a receptive layer, it can mildlypenetrate to be partially embedded within the layer, depending on theoperating conditions and selected materials. Moreover, though theillustrations related to schematic exemplary printed results of thepresent printing method, a layer substantially similar to 810 can beformed on the donor surface 12.

FIGS. 9A to 9C schematically illustrate cross sections of printedconstructions 900 as obtainable using known printing technologies. Foravoidance of a doubt none of the illustrative figures are drawn toscale, such being the case in particular for FIGS. 8A-B and 9A-C, thecomparison being therefore merely qualitative.

FIG. 9A illustrates a representative metallised image resulting frommetal foil printing. In such printed construction an adhesive layer 910is typically transferred with a metal layer 920, so as to permit theattachment of the metal layer to the substrate 20. FIGS. 9B and 9Cillustrate printed constructions prepared with inks comprising acustomary mixture of particles and a binder, such binder-containing inksbeing applied to a printing substrate 20 by printing methods of the art.Layer 930 illustrates the binder film or matrix typically formed on thesubstrate while using such known inks and methods. As can be seen, suchtechniques generally yield strata of particles, the particles in thestrata more distant from the surface of the printing substrate havingoften no possible direct contact with the substrate. Depending on theprinting technique, the materials used therein and the operationcondition, the particles may display patterns falling broadly in twocategories. FIG. 9B schematically illustrates a situation whereparticles 902, having an outer surface 904, display a non-leafingbehavior, the particles being at least partially randomly distributedand/or oriented within the binder matrix. FIG. 9C schematicallyillustrates an alternative situation where particles 906 display aleafing behavior, the particles tending to migrate toward the interfacebetween the binder film and air. Therefore such particles of knownprinted constructions tend to form a gradient of distribution, theirdensity being higher closer to the interface with the air. Leafingparticles are also typically more evenly oriented within the bindermatrix. It is to be noted that in such examples, the surfaces 904 ofparticles 902 need not be hydrophobic. As previously illustrated in thecontext of the inventive print constructions enabled by the presentdisclosure, printed constructions of the prior art can be furtherovercoated (not shown).

In the description and claims of the present disclosure, each of theverbs, “comprise” “include” and “have”, and conjugates thereof, are usedto indicate that the object or objects of the verb are not necessarily acomplete listing of members, components, elements, steps or parts of thesubject or subjects of the verb. These terms encompass the terms“consisting of” and “consisting essentially of”.

As used herein, the singular form “a”, “an” and “the” include pluralreferences and mean “at least one” or “one or more” unless the contextclearly dictates otherwise.

Positional or motional terms such as “upper”, “lower” “right”, “left”,“bottom”, “below”, “lowered”, “low”, “top”, “above”, “elevated”,“vertical”, “horizontal”, “backward”, “forward”, “upstream” and“downstream”, as well as grammatical variations thereof, may be usedherein for exemplary purposes only, to illustrate the relativepositioning, placement or displacement of certain components, toindicate a first and a second component in present illustrations or todo both. Such terms do not necessarily indicate that, for example, a“bottom” component is below a “top” component, as such directions,components or both may be flipped, rotated, moved in space, placed in adiagonal orientation or position, placed horizontally or vertically, orsimilarly modified.

Unless otherwise stated, the use of the expression “and/or” between thelast two members of a list of options for selection indicates that aselection of one or more of the listed options is appropriate and may bemade.

In the disclosure, unless otherwise stated, adjectives such as“substantially” and “about” that modify a condition or relationshipcharacteristic of a feature or features of an embodiment of the presenttechnology, are to be understood to mean that the condition orcharacteristic is defined to within tolerances that are acceptable foroperation of the embodiment for an application for which it is intended,or within variations expected from the measurement being performedand/or from the measuring instrument being used. When the term “about”precedes a numerical value, it is intended to indicate +/−15%, or+/−10%, or even only +/−5%, and in some instances the precise value.

While this disclosure has been described in terms of certain embodimentsand generally associated methods, alterations and permutations of theembodiments and methods will be apparent to those skilled in the art.The present disclosure is to be understood as not limited by thespecific examples described herein.

The invention claimed is:
 1. A method of printing onto a surface of asubstrate, which comprises providing an endless donor surface,cyclically passing the donor surface through a coating station fromwhich the donor surface exits coated with a monolayer of individualparticles after each passage, wherein the particles adhere more stronglyto the donor surface than to one another, and repeatedly performing thesteps of: (i) treating the substrate surface to render the affinity ofthe particles to at least selected regions of the substrate surfacegreater than the affinity of the particles to the donor surface, (ii)contacting the substrate surface with the donor surface to causeparticles to transfer from the donor surface only to the treatedselected regions of the substrate surface, thereby exposing regions ofthe donor surface from which particles are transferred to correspondingregions on the substrate; and (iii) returning the donor surface to thecoating station to render the particle monolayer continuous in order topermit printing of a subsequent image on a substrate surface.
 2. Amethod as claimed in claim 1, wherein the proportion of the particlestransferred to the selected regions of the substrate surface is suchthat a bare surface of the substrate within the selected regions issubstantially imperceptible to the naked eye.
 3. A method as claimed inclaim 1, wherein the proportion of the substrate surface in the selectedregions coated with particles is within the range of 80% to 100%.
 4. Amethod as claimed in claim 1, wherein the proportion of the substratesurface in the selected regions coated with particles is within therange of 50% to 80%.
 5. A method as claimed in claim 1, wherein theproportion of the substrate surface in the selected regions coated withparticles is within the range of 20% to 50%.
 6. A method as claimed inclaim 1, wherein step (i) comprises exposing the selected regions of thesubstrate surface to radiation to activate a receptive layer that ispre-applied to the substrate.
 7. A method as claimed in claim 1, whereinstep (i) comprises applying a receptive layer to the selected regions ofthe substrate surface.
 8. A method as claimed in claim 7, wherein thereceptive layer is applied to the substrate surface by indirectprinting.
 9. A method as claimed in claim 8, wherein the receptive layeris applied to the substrate surface by indirect printing selected fromthe group comprising offset printing, screen printing, flexographicprinting and gravure printing.
 10. A method as claimed in claim 7,wherein the receptive layer is applied to the substrate surface bydirect printing, including by direct jetting.
 11. A method as claimed inclaim 7, wherein the receptive layer applied or activated on thesubstrate surface has a thickness between about 0.5 μm and about 500 μm.12. A method as claimed in claim 7, wherein the receptive layer appliedor activated on the substrate surface has an outer surface distal fromthe substrate, said outer surface being substantially smooth with asurface roughness R_(a) of no more than about 2 μm.
 13. A method asclaimed in claim 7, wherein the receptive layer applied or activated onthe substrate surface is substantially dry upon contacting the particlescoated on the donor surface.
 14. A method as claimed in claim 1, whereinthe particles coated on the donor surface are substantially dry uponcontacting selected regions of the substrate surface.
 15. A method asclaimed in claim 1, wherein the particles comprise a coated or uncoatedmetal, alloy, mica or particles of a polymeric or ceramic materialhaving a metallic appearance or surface reflectivity.
 16. A method asclaimed in claim 1, wherein the particles are flakes or platelet shaped.17. A method as claimed in claim 16, wherein at least 50% of theparticles, or at least 75% of the particles, or at least 90% of theparticles have a thickness not exceeding 100 nm.
 18. A method as claimedin claim 16, wherein at least 50% of the particles, or at least 75% ofthe particles, or at least 90% of the particles have a thickness of atleast 10 nm.
 19. A method as claimed in claim 16, wherein the particleshave an average aspect ratio of at least 10:1, or at least 20:1, or atleast 50:1, or at least 100:1 between at least one of a longestdimension, a mean diameter and an equivalent diameter, as the case maybe, and a thickness of said platelet.
 20. A method as claimed in claim19, wherein said aspect ratio is at most 200:1, or at most 150:1, or atmost 120:1.
 21. A method as claimed in claim 16, wherein the particlestransferred to the substrate surface are disposed upon the receptivelayer in a substantially uniform orientation, the platelets beingessentially parallel to a surface of the receptive layer distal to thesubstrate.
 22. A method as claimed in claim 1, which comprisesprocessing the substrate surface further after having made contact withthe donor surface.
 23. A method as claimed in claim 22, wherein theprocessing comprises burnishing or applying heat and pressure in orderto modify the appearance of the particles adhered to the surface of thesubstrate.
 24. A method as claimed in claim 22, wherein the processingcomprises curing or further curing the receptive layer.
 25. A method asclaimed in claim 22, wherein the processing comprises coating at leastthe selected regions of the substrate surface or the entire surface ofthe substrate with a lacquer.
 26. A method as claimed in claim 1,wherein contacting the substrate surface with the donor surfacecomprises transferring at least a portion of the monolayer of individualparticles from the donor surface to the treated selected regions of thesubstrate surface.